Mech.  dcot 


LIBRARY 

OF  THE 

UNIVERSITY  OF  CALIFORNIA. 


Class 


library  of  the  Department 
OF 

Mechanical  and  Electrical  Engineering 


The  D.  Van  Nostrand  Company 

intend  this  book  to  be  sold  to  the  Public 
at  the  advertised  price,  and  supply  it  to 
the  Trade  on  terms  which  will  not  allow 
of  discount. 


DYNAMO  ELECTRIC 
MACHINERY; 

ITS  CONSTRUCT"^,  DESIGN, 

—At^^.        .  -^V£#A.  , 


DIRECT-CURRENT  MACHINES 


BY 

SAMUEL    SHELDON,  A.M.,  PH.D.,  D.Sc. 

PROFESSOR    OF    PHYSICS    AND    ELECTRICAL    ENGINEERING    AT    THE    POLYTECHNIC 

INSTITUTE    OF    BROOKLYN    AND    PAST-PRESIDENT   OF    THE    AMERICAN 

INSTITUTE    OF    ELECTRICAL    ENGINEERS 

AND 

ERICH   HAUSMANN,  E.E.,  M.S. 

INSTRUCTOR      IN     PHYSICS      AND     ELECTRICAL     ENGINEERING     AT     THE 

POLYTECHNIC  INSTITUTE  OF  BROOKLYN,  AND  ASSOCIATE 

OF     THE     AMERICAN     INSTITUTE     OF 

ELECTRICAL    ENGINEERS 

EIGHTH  EDITION,    COMPLETELY  REWRITTEN 


NEW    YORK: 
D.    VAN    NOSTRAND    COMPANY 

LONDON 

CROSBY    LOCKWOOD   &   SON 
1910 


15?  if 

Engineei  ing- 
Library 


COPYRIGHT,  1900,  BY 
D.  VAN   NOSTRAND  COMPANY 


COPYRIGHT,  1910,  BY 
D.  VAN   NOSTRAND  COMPANY 


Stanhope  fl>ress 

F.    H.   GILSON     COMPANY 
BOSTON.     U.S.A. 


PREFACE. 


THE  object  aimed  at  in  the  preparation  of  the  first 
edition  of  this  book  has  been  kept  in  view  and  has  con- 
trolled the  preparation  of  this  eighth  edition.  This  has 
been  the  production  of  a  text-book  for  the  use  of  students 
pursuing  electrical  or  non-electrical  engineering  courses. 
The  method  of  presentation  is  considered  as  especially 
adapted  for  classroom  exercises,  which  consist  of  recita- 
tions, computations,  and  occasional  lectures,  and  which  are 
supplemented  by  laboratory  exercises,  the  two  being  cor- 
related with  a  view  to  training  the  mind  of  the  student 
and  adding  somewhat  to  his  knowledge.  It  will  be  found 
that  in  treatment  the  sequence  is  such  that  parts  which  it 
may  seem  undesirable  to  require  from  other  than  electrical 
engineering  students  may  be  omitted  without  introducing 
a  discontinuity  in  the  matter  which  remains. 

With  the  exception  of  the  first  two  chapters,  the  book 
has  been  entirely  rewritten;  nearly  two  hundred  of  its  illus- 
trations are  new,  most  of  them  having  been  specially  drawn 
to  make  clear  methods  of  construction  or  characteristics  of 
operation;  and  it  has  been  considerably  extended  in  scope. 
In  the  new  matter  will  be  found  a  set  of  problems  at  the 
end  of  each  chapter,  a  presentation  of  the  theory  of  com- 
mutation, means  for  the  predetermination  of  the  operating 
characteristics  of  direct-current  generators  and  motors,  a 
discussion  on  storage  batteries  from  the  engineering  point 
of  view,  a  treatment  of  the  theory  of  balancers  and  of 

iii 

222638 


iv  PREFACE. 

boosters,  and  a  discussion  of  costs,  prices,  and  operating 
expenses  of  machines  and  plants. 

The  chapters  on  the  design  of  machines  and  on  tests, 
which  appeared  in  the  former  editions,  have  been  omitted,  as 
these  subjects  require  for  their  adequate  treatment  more 
space  than  one  would  be  warranted  in  giving  them  in'  a 
book  of  this  character. 

POLYTECHNIC  INSTITUTE, 
BROOKLYN,  NEW  YORK, 
June   i,   1910. 


CONTENTS. 


CHAPTER  I. 

ELECTRICAL  LAWS  AND  FACTS. 

ART.  PAGE 

1.  Mechanical  Units i 

2.  Electrical  Units 3 

3.  Ohm's  Law 4 

4.  Resistance  of  Conductors 5 

5.  Divided  Circuits 8 

6.  Power  of  Electric  Current 10 

7.  Heat  Developed  by  a  Current 1 1 

8.  Insulating  Materials 1 1 

9.  Test  of  Dielectric  Strength 13 

Problems 16 

CHAPTER  II. 

MAGNETIC  LAWS  AND  FACTS. 

10.  Strength  of  Magnet  Pole 18 

11.  Magnetic  Field  and  Lines  of  Force 18 

1 2.  Intensity  of  Magnetic  Field 19 

13.  Magnetic  Potential 20 

14.  Permeability 20 

15.  Electro-magnetic  Induction 21 

16.  Direction  of  Induced  E.M.F 23 

17.  Inductance 24 

18.  Growth  of  Current  in  an  Inductive  Circuit 26 

19.  Decay  of  Current  in  an  Inductive  Circuit 27 

20.  Quantity  of  Electricity  Traversing  a  Circuit  Due  to  a  Change  of 

Flux  Linked  with  it 29 

21.  Work  Performed  by  a  Conductor  Carrying  a  Current  and  Moving 

in  a  Magnetic  Field 29 

22.  Force  Exerted  between  a  Field  and  a  Conductor  Carrying  a 

Current 3° 

v 


vi  CONTENTS. 

ART.  PAGE 

23.  Magnetomotive  Force  of  a  Circular  Circuit  Carrying  a  Current  31 

24.  The  Toroid 32 

25.  Magnetization  Curves 33 

26.  Reluctance  and  Permeance 36 

27.  Relation  between  Magnetomotive   Force,   Magnetic  Flux,   and 

Reluctance 37 

28.  Hysteresis 38 

29.  Eddy  Currents 42 

Problems 43 


CHAPTER  III. 

ARMATURES. 

30.  Dynamos 45 

31.  Principle  of  Action  of  a  Generator 45 

32.  The  Function  of  the  Commutator 46 

33.  Electromotive  Force  Generated 48 

34.  The  Armature 52 

35.  The  Field  Magnets 53 

36.  Armature  Windings 55 

37.  Multiplex  Armature  Windings 62 

38.  Equalizing  Connections 65 

39.  E.M.F.  Equation  of  Dynamos 66 

40.  Core  Construction 67 

41.  Armature  Coils 73 

42.  Commutators 76 

43.  Brushes  and  Brush  Holders 81 

44.  Shafts  and  Bearings 83 

Problems . .  86 


CHAPTER  IV. 
FIELD  MAGNETS. 

45.  Field-Magnet  Frames 88 

46.  Methods  of  Field  Excitation 92 

47.  Magnetic  Leakage 94 

48.  Calculation  of  Exciting  Ampere-Turns 96 

49.  Field  Coils 104 

Problems .  .                                 108 


CONTENTS.  vii 


CHAPTER  V. 

ARMATURE  REACTION.    COMMUTATION. 

ART.  PAGE 

50.  Armature  Reaction no 

51.  Cross-Magnetizing  Effect  of  Armature  Current in 

52.  Demagnetizing  Effect  of  Armature  Current 113 

53.  Compensation  for  Armature  Reaction 115 

54.  Devices  for  Reducing  Armature  Reaction 118 

55.  Commutation 121 

56.  Time  of  Commutation 125 

57.  Calculation  of  Reactance  Voltage 127 

58.  Conditions  for  Good  Commutation 134 

Problems 138 


CHAPTER  VI. 

GENERATORS. 
Efficiency  of  Operation. 

59.  Capacity  of  a  Dynamo 140 

60.  Heating  of  Dynamos 142 

61.  Output  Coefficients 145 

62.  Losses  in  Armature  Cores 146 

63.  Armature  Copper  Loss 148 

64.  Pole- Face  Losses 149 

65.  Excitation  Loss 151 

66.  Bearing  Friction  and  Windage 151 

67.  Commutator  Loss 152 

68.  Temperature  Elevation. 153 

69.  Efficiency 155 

70.  Coefficient  of  Conversion 158 

71.  Economic  Coefficient 158 

72.  Magnetos 159 

73.  Constant-Potential  and  Constant-Current  Supply 160 

Constant- Potential  Generators. 

74.  Characteristic  Curves  of  Shunt- Wound  Generators 162 

75.  Voltage  Regulation 164 

76.  Hand  Regulation 165 

77.  Field  Rheostats 166 


Vlll  CONTENTS. 

ART.  PAGE 

78.  Self-Regulation 171 

79.  Characteristic  Curves  of  Compound- Wound  Generators 172 

80.  Railway  and  Lighting  Generators 173 

81.  Three- Wire  Generators 180 

82.  Homopolar  Dynamos 184 

Constant-Current  Generators. 

83.  Characteristic  Curves  of  Series- Wound  Generators 187 

84.  Power  Lines 189 

85.  Series- Wound  Generators 189 

86.  The  Brush  Machine 193 

87.  The  Excelsior  Arc-Light  Generator 198 

88.  The  Thomson-Houston  Dynamo 200 

89.  Western  Electric  Arc-Light  Dynamo 204 

Problems 207 


CHAPTER  VII. 

MOTORS. 

90.  Principle  of  Action  of  a  Motor 210 

91.  Direction  of  Rotation 211 

92.  Torque  Exerted  by  a  Motor 213 

93.  Counter  Electromotive  Force 214 

94.  Armature  Reactions 216 

95.  Power  of  Motors 216 

Shunt  Motors. 

96.  Speed  of  Shunt  Motors 217 

97.  Starting  of  Shunt  Motors 225 

98.  Design  of  Starting  Rheostats 230 

99.  Speed  Regulation 232 

100.  Characteristic  Curves  of  Shunt  Motors 233 

101.  Industrial  Applications  of  Shunt  Motors 236 

Series  Motors. 

102.  Series  Motors 239 

103.  Characteristic  Curves  of  Series  Motors 241 

104.  Railway  Motors 241 

105.  Railway  Motor  Control 251 

106.  Motors  for  Automobiles 258 


CONTENTS.  IX 

ART.  PAGE 

107.  Motors  for  Rolling  Mills 260 

108.  Crane  Motors 261 

109.  Compound- Wound  Motors 263 

Problems 265 


CHAPTER  VIII. 

DYNAMOTORS,  MOTOR-GENERATORS,  BOOSTERS,  AND  STORAGE  BATTERIES. 

1 10.   Dynamotors 266 

in.   Motor-Generators 273 

112.  Boosters 279 

1 13.  Storage  Batteries 287 

Problems 292 

CHAPTER  IX. 

CENTRAL-STATION  EQUIPMENT. 

114.  Paralleling  of  Generators 294 

115.  Parallel  Operation  of  Motors 299 

116.  Switches 300 

117.  Fuses 302 

118.  Circuit  Breakers 303 

119.  Measuring  Instruments 305 

120.  Switchboards 312 

121.  Works  Cost 314 

122.  Selling  Prices 315 

123.  Plant  Costs 318 

124.  Operating  Expenses 319 

125.  Cost  of  Electrical  Energy 320 

Problems 322 


DYNAMO  ELECTRIC  MACHINERY. 


CHAPTER    I. 

ELECTRICAL  LAWS  AND  FACTS. 

i.  Mechanical  Units.  —  Force  is  that  which  tends  to 
produce,  alter,  or  destroy  motion.  The  units  of  force  are 
the  dyne  and  the  poundal.  The  dyne  is  that  force  which, 
acting  on  a  one-gram  mass  for  one  second,  will  tend  to 
produce  a  velocity  of  one  centimeter  per  second.  The 
poimdal  is  that  force  which,  acting  on  a  mass  of  one  pound 
for  one  second,  will  tend  to  produce  a  velocity  of  one  foot 
per  second.  The  weight  of  a  pound  mass  is  frequently 
taken  as  a  unit  of  force,  and  is  called,  for  brevity,  a  pound. 
A  force  of  one  pound  is  approximately  equal  to  32.2 
poundals. 

Work  is  the  production  of  motion  against  resistance. 
The  units  of  work  are  the  foot-pound  and  the  erg.  The 
foot-pound  is  the  work  done  in  lifting  a  body  weighing  one 
pound  one  foot  vertically.  The  erg  is  the  work  performed 
by  a  force  of  one  dyne  in  moving  a  body  one  centimeter 
in  the  direction  of  the  force.  T}\Q  joule  is  a  larger  unit 
much  used,  and  is  equal  to  io7  ergs. 

Energy  is  the  capacity  to  do  work.  It  is  expressed  in 
the  same  units  as  work.  The  two  classes  of  energy  are 
Kinetic  energy  and  Potential  energy.  A  body  possesses 


.     hECT  RIC   MACHINERY. 

kinetic  energy  in  virtue  of  its  motion,  while  potential  energy 
is  due  to  the  separation  or  the  disarrangement  of  attracting 
particles  or  masses.  A  wound-up  spring  has  potential 
energy  because  of  the  strained  positions  of  the  molecules, 
while  a  weight  raised  to  a  height  has  potential  energy 
because  of  the  separation  of  its  mass  from  the  attracting 
mass  of  the  earth.  The  potential  energy  of  a  body  is 
measured  by  the  work  required  to  put  the  body  into  its 
strained  condition.  The  kinetic  energy  of  a  body  is  pro- 
portional to  its  mass  and  to  the  square  of  its  velocity,  or 

Wv* 
Kinetic.  Energy  =  -  , 

*g 

since  the  mass  of  a  body  is  equal  to  its  weight  divided  by 
the  acceleration  due  to  gravity.  Kinetic  energy  will  be 
expressed  in  ergs  when  W\s  the  weight  in  grams  of  the 
body  whose  velocity  is  v  centimeters  per  second,  and  when 
g  is  981  cm.  per  second  per  second.  If  Wbe  the  weight 
in  pounds,  v  the  velocity  in  feet  per  second,  and  g  is  32.2 
feet  per  second  per  second,  then  the  kinetic  energy  is  ex- 
pressed in  foot-pounds. 

Power  is  the  rate  of  performance  of  work.  Its  units  are 
the  horse-power  and  the  watt.  A  horse-power  is  33,000 
foot-pounds  per  minute.  A  watt  is  one  joule  per  second. 
One  horse-power  is  equivalent  to  746  watts.  Representing 
the  torque  or  twisting  moment  of  a  machine  in  pound-feet 
by  Ty  and  its  angular  velocity  in  radians  per  second  by  to 
=  2  TT  F/6o,  where  V  is  the  number  of  revolutions  per 
minute,  then  the  horse-power  of  the  machine  is 


H  p 

33000        33000 
In  a  belt-driven  machine  the  torque  in  the  shaft  is  equal  to 


ELECTRICAL    LAWS   AND    FACTS.  3 

the  difference  in  tension  of  the  two  sides  of  the  belt  multi- 
plied by  the  radius  of  the  pulley,  that  is,  T  =  (F  —  F')  r 
pound-feet. 

2.  Electrical  Units.  —  Since  distinction  must  continually 
be  made  between  absolute  or  c.g.s.  units  and  practical  units, 
throughout  this  work  capital  letters  will  be  used  for  quan- 
tities expressed  in  practical  units,  and  lower-case  letters  for 
quantities  expressed  in  absolute  units. 

The  absolute  unit  of  current  is  such  that,  when  flowing 
through  a  conductor  of  one  centimeter  length,  which  is 
bent  into  an  arc  of  one  centimeter  radius,  it  will  exert  a 
force  of  one  dyne  on  a  unit  magnet  pole  (§  10)  placed  at 
the  center.  The  practical  unit  of  current,  the  ampere,  is 
one-tenth  the  magnitude  of  the  absolute  unit. 

The  absolute  unit  of  quantity  is  that  quantity  of  elec- 
tricity which  in  one  second  passes  any  cross-section  of  a 
conductor  in  which  the  absolute  unit  of  current  is  flowing. 
The  practical  unit  of  quantity  is  one-tenth  of  the  absolute 
unit,  and  is  called  the  coulomb.  For  large  quantities  the 
ampere-Jiour  and  the  mega-coulomb,  the  latter  being  equal 
to  a  million  coulombs,  are  units  frequently  used ;  and  for 
small  quantities  the  micro-coulomb,  equal  to  one-millionth 
of  a  coulomb,  is  often  used. 

The  absolute  unit  of  difference  of  potential  exists  between 
two  points  when  it  requires  the  expenditure  of  one  erg  of 
work  to  move  an  absolute  unit  quantity  of  electricity  from 
one  point  to  the  other.  The  practical  unit  of  difference 
of  potential,  the  volt,  is  io8  times  as  large  as  the  absolute 
unit. 

It  is  convenient  and  rational  to  make  a  distinction  be- 
tween electromotive  force  and  difference  of  potential. 
Electromotive  force  is  produced  when  a  conductor  cuts 


4  DYNAMO    ELECTRIC   MACHINERY. 

magnetic  lines  of  force,  or  when  the  electrodes  of  a  pri- 
mary battery  are  immersed  in  a  solution.  But  a  difference 
of  potential  may  exist  due  to  the  flow  of  an  electric  cur- 
rent. Between  any  two  points  of  a  conductor  carrying  a 
current  there  is  that  which  would  send  a  current  through 
an  auxiliary  wire  connecting  these  points,  and  it  is  called 
difference  of  potential.  If  the  current  in  the  original  con- 
ductor be  doubled,  the  difference  of  potential  between  the 
same  two  points  will  be  doubled,  showing  that  this  differ- 
ence of  potential  exists  because  of  the  current  flowing  in 
the  original  conductor.  The  word  pressure  is  used  either 
for  difference  of  potential  or  for  E.M.F.  with  obvious 
relevancy. 

The  absolute  unit  of  resistance  is  offered  by  a  body  when 
it  allows  an  absolute  unit  of  current  to  flow  along  it  be- 
tween its  two  terminals,  when  these  are  maintained  at  unit 
(absolute)  difference  of  potential.  The  practical  unit  of 
resistance,  the  ohm,  is  io9  times  as  large  as  the  absolute 
unit.  The  megohm,  equal  to  a  million  ohms,  and  the 
microhm,  equal  to  one-millionth  of  an  ohm,  are  units  fre- 
quently used. 

3.  Ohm's  Law.  —  The  relation  between  the  current, 
electromotive  force,  and  resistance  of  a  simple  circuit  is 
given  by  Ohm's  law,  in  absolute  units,  as 

e 

t  =  -. 
r 

Since  the  current  in  amperes  is  102,  the  E.M.F.  in  volts 
is  IQ-8^,  and  the  resistance  in  ohms  is  io-9r,  Ohm's  law  may 
be  expressed  in  practical  units  by  the  formula 

:       "'-f     ,         ' 


ELECTRICAL   LAWS   AND    FACTS.  5 

where  7  is  the  number  of  amperes  flowing  in  an  undivided 
circuit,  E  the  algebraic  sum  of  all  the  electromotive  forces 
in  that  circuit  in  volts,  and  R  the  sum  of  all  the  resistances 
in  series  in  that  circuit  expressed  in  ohms. 

The  form  of  the  equation  E  =  IR,  as  applied  to  a  por- 
tion of  a  circuit,  is  much  used  under  the  name  of  Ohm's 
law.  In  this  case,  however,  E  is  not  E.M.F.,  but  differ- 
ence of  potential,  as  explained  in  the  last  article. 

If,  in  a  house  lighted  by  electricity,  the  service  maintains 
a  constant  pressure  of  100  volts  at  the  mains  where  they 
enter  from  the  street,  and  no  lights  be  turned  on,  then  at 
every  lamp  socket  in  the  house  there  will  be  a  pressure  of 
100  volts.  If  now  a  lamp  be  turned  on,  it  will  be  working 
on  less  than  100  volts,  because  of  the  drop  or  fall  of  po- 
tential. If  many  lamps  be  turned  on,  a  considerable  drop 
may  occur.  The  drop  is  caused  by  the  resistance  of  the 
wires  carrying  the  current  from  the  place  of  constant  po- 
tential to  the  place  where  it  is  used,  and  the  volts  lost  have 
been  consumed  in  doing  useless  work,  i.e.  heating  the  wires. 
That  the  drop  is  proportional  to  the  current  flowing  is 
shown  by  a  simple  application  of  Ohm's  law. 

Let  R  be  the  resistance  of  the  line,  and  Ed  the  volts 
drop  caused  thereby  when  a  current  /  flows.  Then 

Ed=IR, 

from  which  it  is  evident  that  the  drop  varies  as  the  cur- 
rent when  the  resistance  of  the  line  is  constant. 

4.  Resistance  of  Conductors.  — The  resistance  R  of  a  con- 
ductor is  expressed  by  the  formula 


f* 

where  p  is  a  constant  called  the  resistivity,  and  depending 


DYNAMO   ELECTRIC  MACHINERY. 


upon  the  material  and  the  temperature  of  the  conductor,  /  is 
the  length,  and  A  the  cross-section  of  the  conductor.     The 

reciprocal  of  the  resistivity,  —  is  called  the  conductivity  of  a 

P 
substance. 

If,  in  the  foregoing  expression  for  R,  the  centimeter  and 
square  centimeter  be  the  units  of  length  and  cross-section 
respectively,  and  the  resistance  is  desired  in  ohms,  then 
p  must  be  the  resistance  between  opposite  faces  of  a  cen- 
timeter cube  of  the  given  material,  and  this  is  called  its 
specific  resistance.  Areas  of  conductors  are  frequently  ex- 
pressed in  terms  of  a  unit,  circular  mil,  equal  to  the  area 
of  a  circle  y^^  inch  in  diameter.  If  area  be  so  expressed 
and  if  /  be  the  length  in  feet  of  the  conductor,  then  p  must 
be  the  resistance  of  a  portion  thereof  one  foot  long  and 
one  circular  mil  in  cross-section,  i.e.  of  one  mil-foot,  so 
that  R  may  be  in  ohms.  The  resistivities  of  various 
metals  at  o°  Centigrade  are  given  in  the  following  table : 

RESISTIVITIES. 


MATERIAL 

SPECIFIC  RESIST- 
ANCE   IN    MI- 
CROHMS   AT   O°C. 

RESISTANCE  PER 
MIL-  FOOT,    IN 
OHMS   AT  0°  C. 

Copper  (soft)     ... 

L594 

2-55 
8.7 
8.98 
13.0 
34 
43 
45 
49 
5° 

II 

9.6 
154 

52.4 

£' 

2O4 

259 
271 

295 

301 
512 

518 

Iron  (soft)     

Platinum       

Steel    .     .     .     

German  Silver  (18%) 

German  Silver  (30%) 

"Advance"  

"lala"    

"Climax"      

"  Superior  "  

As  rectangular  conductors  are  much  used  in  armatures 
and  upon  switchboards,  it  frequently  becomes  necessary  to 

ELECTRICAL   LAWS   AND    FACTS.  7 

express  their  cross-sections  in  circular  mils.  Since  the 
cross-section  of  a  circle  having  a  diameter  of  yoVo  inch  is 
0.00000078  5  square  inches,  the  equivalent  cross-section  of 
a  conductor  expressed  in  circular  mils  is  equal  to  its  cross- 
section  in  square  inches  divided  by  0.000000785,  and 
the  cross-section  in  circular  mils  is  equal  to  1273236  times 
its  cross-section  in  square  inches. 

The  resistivity  of  a  conductor  depends  upon  its  physical 
condition  and  upon  its  purity.  Thus,  for  example,  the 
resistivity  of  soft  copper  is  two  per  cent  lower  than  that  of 
hard  copper.  The  resistivity  of  an  alloy  is  usually  greater 
than  that  of  any  of  its  constituents,  consequently  the  ad- 
mixture of  a  small  percentage  of  one  metal  with  another 
usually  implies  a  higher  resistivity. 

The  American  Institute  of  Electrical  Engineers  has 
adopted  as  its  standard  resistivity  for  soft  copper  one  given 
by  Matthiessen.  A  wire  of  standard  soft  copper,  of  uni- 
form cross-section,  of  one  meter  length,  and  weighing  one 
gram,  should  have  a  resistance  of  0.141729  international 
ohms  at  o°  C.  A  commercial  copper  showing  this  resis- 
tivity is  said  to  have  100  per  cent  conductivity.  Copper  is 
frequently  obtained  having  a  conductivity  of  102  per  cent. 
It  is  in  these  cases  almost  invariably  electrolytic  copper. 

The  resistance  of  conductors  depends  upon  temperature. 
Rise  of  temperature  causes  an  increase  of  resistance  in  all 
pure  metals,  and  the  rate  of  increase  is  approximately  the 
same  for  each.  Representing  the  increase  of  resistance 
per  unit  resistance  at  o°  C.  and  unit  rise  in  temperature  by 
a,  called  the  temperature  coefficient,  and  the  resistance  of  a 
conductor  at  o°  C.  by  RQ,  then  its  resistance  at  any  temper- 
ature T  may  be  expressed  by 

RT=  RQ  (i  +  « 7). 


8  DYNAMO   ELECTRIC   MACHINERY. 

While  it  is  sufficient  for  engineering  purposes  to  consider 
the  temperature  coefficient  of  any  conductor  constant,  it 
should  be  remembered  that  this  coefficient  varies  slightly 
at  different  temperatures. 

The  average  value  of  the  temperature  coefficient  of  cop- 
per is  0.0042,  that  is,  between  any  initial  and  final  temper- 
ature copper  increases  its  resistance  by  0.42  per  cent  of 
its  resistance  at  zero  degrees  Centigrade  for  each  degree 
rise  of  temperature. 

Many  alloys  have  a  very  small  temperature  coefficient, 
and  are  thus  desirable  for  resistances  in  measuring  instru- 
ments. Acid  and  salt  solutions,  carbon,  hard  rubber,  and 
glass  have  negative  temperature  coefficients. 

5.  Divided  Circuits.  —  When  portions  of  an  electric  cir- 
cuit are  connected  in  series,  the  total  resistance  of  the  cir- 
cuit is  equal  to  the  sum  of  the  resistances  of  the  separate 
portions.  To  determine  the  equivalent  resistance  of  a 
number  of  resistances  connected  in  parallel,  let  /  be  the 
current  flowing  in  the  undivided  part  of  the  circuit  shown 


RI 


Fig.  i. 


in  Fig.  I,  and  let  /t  and  /2  be  the  currents  flowing  in  the 
resistances  R^  and  R2  respectively.     Then 


ELECTRICAL   LAWS   AND   FACTS.  9 

and,  since  the  potential  difference,  E,  across  each  branch 
circuit  is  the  same,  by  Ohm's  law 


whence  .  /,  :  /,  :  :        :        • 

The  currents  in  the  branches  of  a  divided  circuit  are  in- 
versely as  the  resistances  of  the  branches. 

If  Re  be  a  single  resistance,  that,  when  substituted  for 

the  shunted  resistances  Rl  and  R2  will  leave  /  unchanged, 

jg 
then  7  =  —  ;   consequently 


or  Re 


resistance  equivalent  to  a  number  of  shunted  resistances 
is  equal  to  the  reciprocal  of  the  sum  of  the  reciprocals  of  the 
separate  resistances. 

The  distribution  of  current  through  the  elements  of  a 
network  of  conductors,  no  matter  how  complex,  may  be 
determined  by  the  aid  of  the  following  two  laws  due  to 
Kirchhoff  :  —  LAW  I.  —  The  algebraic  sum  of  the  currents 
meeting  at  any  point  of  a  netzuork  is  zero.  LAW  II.  — In 
any  mesh  of  a  network  the  algebraic  sum  of  the  IR  drops 
is  equal  to  the  algebraic  sum  of  the  electromotive  forces. 
For  example,  if  E  be  the  electromotive  force  of  the  battery 
indicated  in  Fig.  i,  according  to  the  first  law 

7-7^=0,  (i) 


10  DYNAMO   ELECTRIC   MACHINERY. 


and  from  the  second  law 

E  =  IR  +  7^ 

and 

E  =  IR  +  IJ 

From  (2)  and  (3) 

^  =  A*,- 

Hence  from  (i) 

/-A-^¥ 

R* 

JP 

or 

L  =7      Ki 

1     *»+.«; 

(2) 
(3) 


=  o, 


6.  Power  of  Electric  Current.  —  If  a  difference  of  poten- 
tial of  e  absolute  units  exist  between  two  points,  the  trans- 
fer of  an  absolute  unit  quantity  of  electricity  from  one 
point  to  the  other  requires  the  expenditure  of  e  ergs  of 
work.  Since  the  volt  is  equal  to  io8  absolute  units  of 
potential  difference,  and  the  coulomb  is  equal  to  io-1  abso- 
lute units  of  quantity,  it  follows  that  the  work  performed  in 
transferring  one  coulomb  of  electricity  under  a  difference  of 
potential  of  one  volt  is  io7  ergs  or  one  joule.  A  current 
of  7  amperes  flowing  for  /  seconds  represents  It  coulombs 
of  electricity,  and  if  these  be  transferred  under  a  potential 
difference  of  E  volts,  the  work  done  in  joules  will  be 

W  =  Bit. 
The  rate  of  working,  or  the  power,  is 

P-Z-EI, 

where   P  is   expressed   in   watts,   i.e.   joules   per    second. 
Since,  from  Ohm's  law,  E  =  IR,  by  substitution 

P  =  PR. 

For  commercial  currents  and  voltages  the  watt  is  a  need- 
lessly small  unit,  hence  the  kilowatt  (=  1000  watts)  is 


ELECTRICAL    LAWS    AND    FACTS.  II 

generally  used  to  express  electrical  power.  It  is  repre- 
sented by  the  abbreviation  K.W.  The  horse-power  is  equal 
to  746  watts,  or  approximately  three-fourths  of  a  K.W. 

7.  Heat  Developed  by  a  Current.  —  When  a  current  7 
is  maintained  in  a  circuit  of  resistance  7?,  the  work  per- 
formed is   converted  into  heat.     The  work  thus  done  per 
second,  or  the  power  expended,  will  be  PR  watts.     Since 
this  production  of  heat  is  often  of  no  service,  this  expendi- 
ture of  power  is  generally  called  the  PR  loss. 

This  production  of  heat  causes  a  rise  of  temperature  in 
the  conductor,  and  the  temperature  will  continue  to  rise  till 
the  heat  generated  per  second  by  the  PR  loss  is  exactly 
counterbalanced  by  the  rate  of  dissipation  of  heat  by  con- 
duction, convection,  and  radiation. 

The  inherent  resistances  of  electrical  machines  involve 
the  production  of  heat  in  their  operation  (as  do  also  fric- 
tion and  reversal  of  magnetism),  which  causes  a  rise  of  tem- 
perature. As  insulating  materials  can  survive  only  moder- 
ately high  temperatures,  such  machines  must  be  designed 
to  operate  without  becoming  too  hot.  This  is  accomplished 
by  decreasing  the  PR  loss,  by  increasing  the  radiating  sur- 
face, and  by  improving  ventilation. 

8.  Insulating  Materials.     The  desirable  properties   of 
materials  which  are  to  be  used  for  insulating  various  elec- 
trical conductors  from  each  other  are  :  (a)  a  high  insulation 
resistance  and  this  resistance  should  remain  high  over  a 
considerable  range  of  temperature  ;  (b)  a  dielectric  strength 
sufficient  to  preclude  any  possibility  of  perforation  by  volt- 
ages  liable  to   exist  between  the  conductors  which  they 
separate,  and  this  strength  must  also  persist  throughout  all 
probable  ranges  of  temperature ;  (c)  such  physical  proper- 
ties  as  will  permit  of  mechanical  manipulation ;  (d)  non- 


12  DYNAMO   ELECTRIC  MACHINERY. 

alteration  of  chemical  constitution  when  subjected  to  high 
temperatures  and  operating  conditions. 

As  no  one  insulating  material  possesses  all  of  these  desir- 
able properties,  for  any  particular  purpose  that  insulating 
material  should  be  chosen  which  is  best  suited  for  the  given 
conditions.  In  this  choice,  available  space  and  cost  of  the 
insulation  are  also  determining  factors. 

The  dielectric  strength  of  an  insulating  material  is 
measured  by  the  voltage  which  must  be  applied  to  it  in 
order  to  cause  its  rupture.  The  dielectric  strength  depends 
upon  the  thickness  of  the  dielectric,  the  form  of  the  opposed 
conducting  surfaces,  and  the  manner  in  which  the  E.M.F. 
is  applied,  whether  gradually,  suddenly,  or  periodically 
varying.  It  has  been  stated  that  the  dielectric  strength 
approximately  varies  inversely  as  the  cube  root  of  the 
thickness,  showing  that  a  thin  sheet  is  relatively  stronger 
than  a  thick  one  of  the  same  material.  For  example,  the 
dielectric  strength  of  mica  when  I  mm.  thick  is  610  kilovolts 
per  centimeter,  but  when  o.i  mm.  thick  it  is  1150  kilovolts 
per  centimeter. 

Mica  possesses  the  highest  insulation  resistance  and  the 
greatest  dielectric  strength  of  insulating  materials.  It  does 
not  absorb  moisture  and  its  chemical  constitution  is  un- 
affected by  high  temperatures.  It  is  not,  however,  me- 
chanically strong. 

Sheets  of  insulation  made  up  from  pieces  of  scrap  mica 
cemented  together  by  linseed  oil  or  preparations  of  shellac, 
when  carefully  constructed  with  lapped  joints,  exhibit 
nearly  as  good  insulating  and  dielectric  properties  as  sheet 
mica.  While  not  perfect  mechanically,  these  sheets  permit 
of  bending  better  than  pure  mica. 

For  insulating   purposes  where    considerable   flexibility 


ELECTRICAL    LAWS   AND    FACTS.  13 

is  essential,  micanite  paper  and  micanite  cloth  are  well 
adapted.  These  materials  consist  of  small  pieces  of  mica 
in  combination  with  paper  or  various  kinds  of  fabric. 

Preparations  of  fibrous  materials  with  linseed  oil,  which, 
after  being  dried,  have  been  thoroughly  baked,  are  fairly 
good  insulators.  As  water  is  generally  present  in  their 
pores,  their  insulation  resistance,  upon  heating,  decreases 
until  the  temperature  has  reached  100°  C,  and  then  it  in- 
creases. These  preparations  are  mechanically  flexible. 
Preparations  of  fibrous  material  with  shellac  are  good  in- 
sulators, but  crack  upon  bending. 

Vulcanized  fibers  are  made  by  treating  paper  fiber  chemi- 
cally, and,  when  dried,  they  have  a  fairly  high  insulation 
resistance,  but  they  readily  absorb  moisture,  and,  upon  dry- 
ing, are  liable  to  warp  and  twist.  They  furthermore  be- 
come brittle  when  heated. 

Hard  rubber  is  a  good  insulator  and  possesses  the  desir- 
able mechanical  qualifications,  but  it  does  not  withstand 
moderately  high  temperatures,  for  at  70°  C.  it  becomes 
soft  and  melts  at  80°  C.  Its  employment  is  limited, 
therefore,  to  apparatus  to  be  used  at  comparatively  low 
temperatures. 

Asbestos,  a  fairly  good  insulator,  is  used  principally  be- 
cause of  its  incombustibility.  Vulcabeston,  which  is  a 
preparation  of  asbestos  and  rubber,  exhibits  good  insulating 
and  mechanical  qualities,  and  is  especially  fitted  for  higher 
temperatures.  Asbestos  and  vulcabeston  are  much  used 
in  electric  heating  apparatus. 

9.  Test  of  Dielectric  Strength.  —  In  order  to  test  the 
voltage  necessary  to  break  down  a  sample  sheet  of  insulat- 
ing material,  the  sample  is  placed  between  two  flat  metal- 
lic surfaces  which  are  connected  respectively  with  the  two 


DYNAMO   ELECTRIC    MACHINERY. 


terminals  of  a  high-voltage  transformer,  whose  voltage  can 
be  varied  at  will.  The  sample  should  project  considerably 
beyond  the  edges  of  the  metallic  surfaces,  so  that  no  dis- 
charge can  take  place  from  one  terminal  to  the  other  around 
the  sample  under  test.  The  test  voltage  is  applied  and  is 
gradually  increased  until  the  material  punctures. 

Practical  average  values  of  dielectric  strengths  over  the 
thicknesses  stated  of  various  insulating  materials  are  given 
in  the  following  table  : 


MATERIAL 

THICKNESS 
IN    MM. 

DIELECTRIC 
STRENGTH    IN 
EFFECTIVE 
SINUSOIDAL 
KILOVOLTS 
PER   CM. 

Air  

O    C 

ZO.T. 

I.O 

43-6 

44 

C.O 

7  J.C 

u 

IO  O 

Asbestos       "... 

o.s-i-1; 

25-10 

Cotton                          .                . 

O  I—  O  "} 

I  IO 

Glass  .                                               .... 

U.i      U.J 

2  O—  O.O 

250—170 

Hard  rubber           .          

i-1?—  7  l.o 

510—400 

Mica  

o.oc-^.o 

I  ^OO—  5OO 

O.2—  O.Z 

400 

"          paper  . 

2 

O.2-O.O 

i  so 

"         cloth    

O.2—  O.6 

80 

Paper  (paraffined)  

O.O5—O.2 

360 

Silk      

O.O2—  O.2 

200-150 

Vulcabeston      

I.O-2.5 

80-20 

Vulcanized  fiber     

O.8-2.O 

80 

For  measuring  the  test  voltage  a  spark  gap  having  sharp 
needle-point  terminals  is  connected  in  parallel  with  the 
sample  under  test.  The  distance  between  the  needle-points 
is  adjustable  so  as  to  limit  the  voltage  which  can  be  im- 
pressed upon  the  conductors  on  each  side  of  the  insulating 
material.  In  carrying  out  the  test,  the  needle-points  are 
adjusted  at  a  certain  minimum  distance  apart.  The  voltage 


ELECTRICAL    LAWS   AND    FACTS.  1$ 

impressed  upon  the  terminals  is  raised  until  a  spark  passes 
between  the  points.  The  air  gap  is  then  increased  in 
length,  and  the  operation  repeated  until  the  sample  breaks 
down,  and  the  spark  passes  through  it  instead  of  across 
the  air  gap.  The  length  of  the  air  gap  is  measured  and  the 
break-down  voltage  may  then  be  obtained  from  the  curve 
of  Fig.  2,  which  shows  the  effective  sinusoidal  voltages 


275 
250 
225 
200 

0 

>150 

_j 

100 
75 
50 
25 

X 

X 

/ 

X 

X 

X 

x 

X 

/ 

^ 

X 

/ 

' 

/ 

/ 

/, 

10       12       14       16 
INCHES 

Fig.  2. 


18       20       22       2-4      26 


corresponding  to  the  sparking  distances   in   air  between 
opposed  sharp  needle-points. 

The  test  voltage  to  be  applied  in  determining  the  suita- 
bility of  insulation  in  commercial  apparatus  depends  upon 
the  kind  and  the  size  of  the  apparatus,  its  normal  voltage, 
and  the  service  for  which  it  is  designed.  The  following 
voltages  for  testing  insulation  of  apparatus  and  cables  by  a 


i6 


DYNAMO   ELECTRIC   MACHINERY. 


continuous  application  for  one  minute  are  recommended  by 
the  American  Institute  of  Electrical  Engineers : 


RATED   TERMINAL  VOLTAGE   OF   CIRCUIT 

RATED   OUTPUT 

TESTING  VOLTAGE 

Not  exceeding  400  volts    .... 
400  volts-8oo  volts  

Under  10  K.W. 
10  K.W.  and  over 
Under  10  K.W. 

1000  volts 
1500      " 
i  noo     " 

loKW  and  over 

2000     " 

800  volts-  1  200  volts      

1  2OO  VOltS-25OO  VOltS    

Any 

Any 
Any 

3500    « 
5000     " 
Double  normal 

rated  voltage 

The  test  voltage  should  be  applied  successively  between 
each  electric  circuit  and  surrounding  conductors  and  also 
between  adjacent  electric  circuits.  High-voltage  tests 
should  be  made  at  the  temperatures  assumed  during  normal 
operation. 

PROBLEMS. 

1.  How  much  work  is  done  by  a  pump  in  raising  2500  gal- 
lons of  water  from  a  mine  200  feet  deep?     If  this  is    accom- 
plished in  25  minutes,  what  is  the  power  of  the  pump  expressed 
in  horse-power  ? 

2.  Calculate  the  kinetic  energy  of   an  electric   locomotive 
weighing  95  tons  when  running  at  a  velocity  of  50  miles  per 
hour. 

^  3.  The  hot  resistance  of  a  loo-candle-power  carbon  incan- 
descent lamp  is  45  ohms.  How  much  current  does  the  lamp 
take  when  connected  to  i2o-volt  mains  ? 

s  4.  What  must  be  the  E.M.F.  of  a  generator  to  supply  a  group 
of  50  lamps  connected  in  parallel,  each  requiring  ^  ampere, 
the  resistance  of  the  generator  being  o.i  ohm,  so  that  each  lamp 
of  220  ohms  resistance  shall  receive  its  full  current,  assuming 
the  line  wires  to  have  a  resistance  of  0.5  ohm  ? 


PROBLEMS.  17 

5.     Determine  the  resistance  of  one  mile  of  copper  wire  0.325 
inch  in  diameter  at  zero  degrees  Centigrade. 
„  6.     Calculate  the  resistance  at  i5°C.  of  a  mile  of  track  rail 
weighing  70  pounds  per  yard,  taking  120  ohms  as  its  resistance 
per  mil-foot  at  that  temperature.     Specific  gravity  of  track  rail 

=  7.8- 

,,  7.  Find  the  resistance  at  700°  C.  of  a  platinum  wire  two 
meters  long  and  one  millimeter  in  diameter  ;  the  temperature 
coefficient  being  0.0036. 

8.  When  four  conductors  of  4,  8,  10,  and  16  ohms  resistance 
respectively  are  joined  in  parallel  to  the  terminals  of  a  battery 
whose  E.M.F.  is  20  volts  on  open  circuit  and  whose  internal 
resistance    is   3   ohms,   how  much  current   will   flow  in    each 
conductor  ? 

9.  Resistances  of  2,  3,  4,  6,  8,  and  10  ohms  respectively  are 
connected  as  shown  in    Fig.  3,  the  number   adjacent  to  each 

vww 


Fig.  3. 

branch  representing  its  resistance.  What  will  be  the  potential 
difference  across  each  resistance  and  the  current  therein  when 
the  terminals  A,  B  are  connected  to  5o-volt  mains  ? 

10.  What  power  is  expended  in  the  loo-candle-power  lamp 
of  problem  3  ?  Express  the  result  in  watts  and  in  horse-power. 
When  electrical  energy  costs  10  cents  per  kilowatt-hour,  how 
much  does  it  cost  to  operate  the  lamp  for  three  hours  ? 


l8  DYNAMO   ELECTRIC    MACHINERY. 


CHAPTER   II. 

MAGNETIC    LAWS    AND    FACTS. 

Strength  of  Magnet  Pole.  —  A  unit  magnet  pole  is 
one  which  will  repel  an  equal  like  pole,  when  at  a  distance 
of  one  centimeter  in  vacuum  or  in  air,*  with  a  force  of  one 
dyne. 

It  follows  from  this  definition  that  a  pole  m  units  strong 
will  repel  a  like  unit  pole  with  a  force  of  m  dynes.  The 
force  exerted  between  two  magnetic  poles  varies  inversely 
as  the  square  of  the  distance  between  them.  Hence  the 
force  in  dynes  exerted  between  two  magnetic  poles  of 
strengths  m  and  m'  when  d  centimeters  apart  in  air  is 
defined  by  the  equation 

•     mm' 

ii.  Magnetic  Field  and  Lines  of  Force.  —  The  space 
around  a  magnet  where  its  action  is  felt  is  termed  the  field 
of  that  magnet.  This  field  may  conveniently  be  considered 
as  permeated  by  lines  of  force.  These  lines  represent  by 
their  direction  the  direction  of  the  force  exerted  by  the 
magnet,  and  by  their  closeness  to  each  other  show  the 
magnitude  of  this  force. 

The  directions  of  the  lines  of  force  in  the  vicinity  of  a 
magnet  may  be  demonstrated  by  scattering  iron  filings 
over  a  glass  plate  laid  on  a  magnet.  Magnetic  poles  are 

*  In  this  chapter  air  is  considered  to  have  the  same  magnetic  properties 
as  a  vacuum. 


MAGNETIC  LAWS   AND    FACTS.  19 

induced  in  the  iron  particles  and  the  latter  arrange  them- 
selves parallel  to  the  lines  of  force,  this  arrangement  being 
facilitated  by  gently  tapping  the  glass  plate. 

12.  Intensity  of  Magnetic  Field.  —  A  magnetic  field  in 
air  is  said  to  have  unit  strength  or  intensity  at  any  point 
therein  when  a  unit  magnet  pole  placed  at  that  point  is 
acted  upon  by  a  force  of  one  dyne,  or  when  a  magnet  pole  m 
units  strong  is  acted  upon  by  a  force  of  m  dynes.  There- 
fore the  strength  of  a  magnetic  field  in  air,  represented  by 
3C,  may  be  expressed  as 

00  =  ^. 
m 

By  convention  one  line  of  force  per  square  centimeter  is 
considered  to  represent  a  field  of  unit  strength,  the  square 
centimeter  being  on  a  surface  that  is  at  all  points  perpendic- 
ular to  the  lines  cutting  it.  Hence  the  strength  or  inten- 
sity of  any  field  in  air,  3C,  can  be  expressed  by  the  number 
of  lines  of  force  per  square  centimeter. 

Suppose  a  sphere  of  one  centimeter  radius  to  be  circum- 
scribed about  a  unit  magnet  pole.  Another  unit  pole  at 
any  point  on  the  surface  of  this  sphere  will  be  acted  upon 
by  a  force  of  one  dyne.  Hence  there  exists  unit  field  in- 
tensity at  any  point  on  this  surface.  But  there  are  4  n 
square  centimeters  on  this  surface,  and  each  square  centi- 
meter will  be  cut  by  one  line  of  force.  Therefore,  there 
emanate  from  a  2init  magnet  pole  4  71  lines  of  force.  Similarly 
a  magnet  pole  of  strength  m  sends  out  4  nm  lines  of  force. 
The  total  number  of  lines  of  force,  or  the  total  magnetic 
flux,  represented  by  the  symbol  <£,  may  therefore  be  ex- 
pressed as  $  =  4  Tim. 

When  a  magnetic  field  has  different  intensities  at  various 
points  in  it,  as  is  usually  the  case,  it  is  called  a  non-uniform 


20  DYNAMO   ELECTRIC   MACHINERY. 

field  ;  and  when  it  has  everywhere  the  same  intensity  and 
direction,  it  is  said  to  be  a  uniform  field. 

13.  Magnetic    Potential.  —  The    magnetic    potential    at 
any  point  is  measured  by  the  work  that  would  be  required 
to  bring  a  unit  magnet  pole  up  to  that  point  from  an  in- 
finite distance. 

The  difference  of  magnetic  potential  between  any  two 
points  is  measured  by  the  work  in  ergs  required  to  carry  a 
unit  magnet  pole  from  one  to  the  other. 

14.  Permeability.  —  The  maintenance  of  the  same  dif- 
ference of  magnetic  potential  between  two  points  will  result 
in  more  lines  of  force  in  iron  than  in  air.     Iron  is  then  said 
to  be  more  permeable  than  air,  or  to  have  a  greater  per- 
meability.    If,  for  a  given  gradient  of  magnetic  potential, 
JC  lines  of  force  per  square  centimeter  be  set  up  with  air 
as  a  medium,  and  later  at  the  same  point  (B  lines  with  some 
other  substance  as  a  medium,  then  the  ratio  of  (B  to  3C 
expresses  the  permeability  of  that  substance.     This  ratio 
is  usually  represented  by  the  symbol  /*,  so  that 

(B 


The  permeability  //  expresses,  therefore,  the  relative  mag- 
netic conductivity  of  a  substance  compared  with  air. 

The  total  flux  in  the  second  substance  is  sometimes 
considered  to  be  made  up  of  two  parts,  the  first  consisting 
of  that  which  would  be  present  with  air  as  a  medium  and 
the  second  being  that  which  is  added  by  the  second 
substance.  The  total  number  of  lines  of  force  per  square 
centimeter,  (B,  produced  in  a  substance  of  permeability  /*  is 
called  the  flux  density,  or  the  induction  per  square  centi- 
meter. For  air,  vacuum,  and  most  substances  /*  =  I  .  For 


MAGNETIC   LAWS   AND  FACTS.  21 

iron,  nickel,  and  cobalt  /*  has  a  higher  value,  reaching,  in 
the  case  of  iron,  as  high  as  3000.  Such  substances  are 
said  to  be  paramagnetic,  or  simply  magnetic.  Bismuth, 
antimony,  phosphorus,  and  a  few  other  materials  have  a 
permeability  very  slightly  less  than  unity  ;  these  being 
known  as  diamagnetic  substances.  A  substance  for  which 
fi  is  zero  would  insulate  magnetism;  but  no  such  substance 
is  known. 

One  line  of  force,  that  is,  a  flux  equal  to  —  that  from  an 

47T 

isolated  unit  pole,  is  called  a  maxwell.  A  flux  density  of 
one  line  of  force,  or  one  maxwell,  per  square  centimeter,  is 
called  a  gauss.  The  total  magnetic  flux,  $,  in  maxwells, 
which  passes  through  an  area  of  A  square  centimeters, 
in  which  the  flux-density  is  (B  gausses,  is  given  by  the 
equation  <j>  =  (g^ 

15.  Electro-Magnetic  Induction. —  In  1831  Faraday  dis- 
covered that  when  a  conductor  was  moved  in  a  magnetic 
field,  an  electromotive  force  was  set  up  in  the  conductor. 
This  phenomenon  is  the  foundation  of  all  modern  electri- 
cal engineering. 

An  absolute  unit  of  E.M.F.  is  produced  when  a  conduc- 
tor cuts  one  line  of  force  per  second.  If  the  conductor 
cuts  two  lines  in  the  second,  or  one  line  in  half  a  second, 
then  two  such  units  of  electromotive  force  are  produced. 
An  E.M.F.  of  one  volt  is  produced  by  the  cutting  of 
io8  lines  of  force  per  second. 

If,  in  the  short  interval  of  time,  dt  seconds,  d$  lines  be 
cut,  then  during  that  interval  the  value  of  the  induced 

E.M.F.  will  be 

d$ 

e  = —  absolute  units, 

dt 


22 


DYNAMO   ELECTRIC   MACHINERY. 


or,  _  i    d$     . 

E  = -volts, 

io8  dt 

the  negative  sign  being  used  because  the  induced  E.M.F. 
tends  to  send  a  current  in  such  a  direction  as  to  demag- 
netize the  field.     When  of  no  con- 
sequence  the    negative   sign   will 
hereafter  be  omitted. 

If  a  conductor,  Fig.  4,  /  centi- 
meters long  moves  in  a  direction 
perpendicular  to  itself  with  a  uni- 
form velocity  of  v  centimeters  per 
second  across  a  uniform  magnetic 
field  having  a  flux  density  of  (B 
gausses,  the  plane  of  its  path 
making  an  angle  a  with  the  direc- 
tion of  the  lines  of  force,  then  the 
number  of  lines  cut  per  second  is  ($>lv  sin  a,  and,  since  the 
rate  of  cutting  is  uniform,  the  E.M.F.  at  any  instant  is 

e'  =  ($>lv  sin  a. 

If  there  be  a  non-uniformity  in  the  rate  of  cutting  lines, 
due  either  to  an  uneven  field  or  to  an  irregular  motion, 
then  the  average  value  of  the  induced  E.M.F.  associated 
with  the  cutting  of  <I>  lines  in  the  time,  /  seconds,  will  be 

eav=  — absolute  units,  or  E&v=  — —  volts. 
t  io  t 

If  a  circular  loop  of  wire  revolve  about  its  diameter  as 
an  axis  in  a  non-uniform  magnetic  field  with  a  constant 
angular  velocity,  or  if  it  revolve  in  a  uniform  field  with  a 
variable  velocity,  its  sides  cut  lines  of  force  at  various  rates. 
The  instantaneous  E.M.F.  in  the  whole  loop  will  be  as 
before,  ^ 


MAGNETIC   LAWS   AND    FACTS. 


where  <J>  is  the  number  of  lines  that  links  with,  or  that 
passes  through,  the  loop.  If  the  loop  be  of  n  turns,  then 
the  pressure  will  be  n  times  as  great,  or  during  the  inter- 
val dt, 

E  = volts. 

l&dt 

1 6.  Direction  of  Induced  E.M.F.  —  The  direction  of 
flow  of  a  current  induced  in  a  closed  circuit  by  moving  it 
in  a  magnetic  field  is  best  represented  by  drawing  the 
conventional  representation  of  the  three  dimensions  of 


Motion 


Motion 


Fig.  5. 


Fig.  6. 


space.  If  the  flux  be  directed  upwards,  and  the  motion  of 
tfye  conductor  be  to  the  right,  then  the  E.M.F,  will  tend 
to  send  a  current  toward  the  reader.  If  any  one  of  these 
conditions  be  changed  it  necessitates  the  change  of  one  of 
the  others,  and  conversely  the  change  of  any  two  leaves 
the  |#ird  unaltered.  About  the  same  idea  is  represented 
in  E^rning's  Rule,  which  is  as  follows  :  — 


24  DYNAMO   ELECTRIC   MACHINERY. 

Let  the  index  finger  of  the  right  hand  point  in  the  di- 
rection of  the  flux,  and  the  thumb  in  the  direction  of  the 
motion.  Bend  the  second  finger  at  right  angles  with  the 
thumb  and  index  finger,  and  it  will  point  in  the  direction 
of  the  E.M.F. 

Another  rule  is  :  — 

Stand  facing  a  north  magnetic  pole.  Pass  a  conductor 
downward.  The  current  tends  to  flow  to  the  left. 
V  17.  Inductance.  — An  electric  current  produces  a  mag- 
netic field  in  the  vicinity  of  the  conductor  carrying  that 
current.  The  conductor  may  therefore  be  considered  as 
encircled  by  lines  of  force.  When  the  current  is  first 
started  in  such  a  conductor,  these  lines  of  force  must  be 
established.  In  establishing  itself,  each  line  is  considered 
as  having  cut  the  conductor,  or,  what  is  equivalent  thereto, 
been  cut  by  the  conductor.  This  cutting  of  lines  of  force 
results  in  the  production  of  an  electromotive  force  in  the 
conductor,  called  the  E.M.F.  of  self-induction.  When  the 
flow  of  current  ceases,  the  surrounding  lines  of  force  col- 
lapse, cutting  the  conductor,  thus  also  producing  an  electro- 
motive force  of  self-induction.  The  E.M.F.  of  self-induction 
is  always  a  counter  E.M.F.,  that  is,  its  direction  is  such 
as  to  tend  to  prevent  the  change  of  current  which 
causes  it. 

The  magnitude  of  this  E.M.F.  is  dependent  upon  the 
rapidity  with  which  the  field  is  established  or  destroyed, 
and  upon  a  constant  called  the  self -inductance  or  the  co- 
efficient of  self-induction  of  the  -circuit.  It  is  generally 
represented  by  the  letter  Z,  and  is  that  coefficient  by  which 
the  time  rate  of  change  of  current  in  the  circuit  must  be 
multiplied  in  order  to  give  the  E.M.F.  induced  in  the  cir- 
cuit. Its  absolute  value  is  numerically  represented  by  the 


MAGNETIC    LAWS   AND    FACTS.  2$ 

number  of  lines  of  force  linked  with  the  circuit  per  absolute 
unit  of  current  in  that  circuit.  Its  practical  unit  is  io9 
times  as  large  as  the  absolute  unit,  and  is  called  the  henry. 
A  circuit  having  an  inductance  of  one  henry  will  have  a 
pressure  of  one  volt  induced  in  it  by  a  uniform  change  of 
current  of  one  ampere  per  second.  Hence  the  E.M.F.  of 
self-induction  may  be  written 

E-      -L*L- 

J-^S    —  •*-'        , 

dt 

But,  from  §15,  the  E.M.F.  induced  in  a  loop  of  wire  mov- 
ing in  a  magnetic  field  is 


^ 

E  =  --    -io~8. 
dt 

Equating  these  expressions,  there  results 


Two  circuits  may  exercise  a  mutually  inductive  action 
upon  each  other,  and  an  E.M.F.  may  be  induced  in  one  by 
a  change  of  current  in  the  other.  This  is  called  an  E.M.F. 
of  mutual  induction.  In  magnitude  it  depends  upon  the 
shape  and  position  of  the  two  circuits,  and  upon  the  char- 
acter of  medium  in  which  they  are  placed.  It  is  also 
dependent  upon  a  constant  which  is  called  the  mutual 
inductance  or  coefficient  of  mutual  induction  of  the  two 
circuits.  It  is  generally  represented  by  the  letter  M. 
It  is  that  coefficient  by  which  the  time  rate  of  change  of 
the  current  in  one  of  the  circuits  is  multiplied  in  order  to 
give  the  E.M.F.  induced  in  the  other  circuit.  Its  absolute 
value  is  numerically  equal  to  the  number  of  lines  of  force 
linked  with  one  of  the  circuits  per  absolute  unit  of  current 


26  DYNAMO   ELECTRIC   MACHINERY. 

in  the  other  circuit.  Its  practical  unit  is  the  same  as  the 
practical  unit  of  self-inductance,  that  is,  the  henry,  and  is 
io9  times  as  large  as  the  absolute  unit. 

1 8.  Growth  of  Current  in  an  Inductive  Circuit.  —  When 
a  current  is  started  in  a  circuit  having  resistance  and  in- 
ductance by  impressing  a  constant  E.M.F.  upon  its  ter- 
minals, the  self-induced  pressure  in  that  circuit  tends  to 
oppose  the  flow  of  the  current  and  prevents  it  reaching  its 
ultimate  value  immediately.  At  the  instant  of  closing  the 
circuit  there  is  no  current  flowing,  and  let  time  be  reckoned 
from  this  instant.  At  any  subsequent  instant,  t  seconds 
later,  the  impressed  E.M.F.  may  be  considered  as  the  sum 
of  two  parts,  E^  and  Er.  The  first,  E^  is  that  part  which 
is  opposed  to,  and  just  neutralizes,  the  E.M.F.  of  self- 
induction,  so  that  El  =  —  Es ;  but 

"     B.O-& 

dt 

so 

dl 


The  second  part,  Er,  is  that  which  is  necessary  to  send 
current  through  the  resistance  of  the  circuit,  according  to 
Ohm's  law,  so  that 

Er=RI. 

The  impressed  electromotive  force,  being  the  sum  of  El 
and  Er,  is  therefore 

E=RI  +  L^-; 

dt 

whence 

1  =  E  -  RI        =      ~~R  '    E-  RI   ' 


MAGNETIC   LAWS   AND    FACTS. 


Integrating  from  the  initial  conditions  t  =  o,  /  =  o,  to  any 
condition  /  =  /•,/  =  /', 

L 


Therefore 


Rt      .       (E-Rr\ 
--=loge(-         -V 


E   r 

from  which  the  instantaneous  current  value  is 


GROWING  CURRENT 
(DIRECT)  E.M.F.=  100 
R  =  10 

L  =   .2 


where  e  is  the  base  of  the  natural  system  of  logarithms 
and  numerically  equal  to  2.7183.  This  equation  shows 
that  the  rise  of  current  in  an  inductive  circuit  follows  a 
logarithmic  curve,  and  that,  when  /  is  of  sufficient  magni- 
tude to  render  the  second  term  negligible,  the  value  of  the 
current  will  be  as  given  by  Ohm's  law,  a  condition  which 
agrees  with  experimental  ob- 
servations. 

A  curve  of  the  growth  of 
current  in  a  circuit  having 
resistance  and  inductance  is 
shown  in  Fig.  7,  the  values 
of  /  being  calculated  for  the 
conditions  noted. 

The  natural  logarithms  used 
in  preceding  formulae  can  be  obtained  by  multiplying  the 
common  logarithm  of  the  number,  the  mantissa  and  charac- 
teristic being  included,  by  2.3026. 

19.  Decay  of  Current  in  an  Inductive  Circuit. —  If  a 
current  be  flowing  in  a  circuit  having  inductance  and  re- 
sistance, and  the  supply  of  E.M.F.  be  discontinued,  with- 


1     .02     .03    .04     .05     .06     .07    .08     .09      .1 
SECONDS 


Fig.  7. 


28 


DYNAMO   ELECTRIC   MACHINERY. 


out,  however,  interrupting  the  continuity  of  the  circuit,  the 
flow  of  current  will  not  cease  instantly,  but  the  E.M.F.  of 
self-induction  will  keep  it  flowing  for  a  time,  with  values 
decreasing  according  to  a  logarithmic  law. 

An  expression  for  the  value  of  this  current  at  any  time, 
/  seconds,  after  withdrawing  the  impressed  E.M.F.  ,  may  be 
obtained  as  in  the  foregoing  section.  The  current  at  the 
instant  of  interruption  of  the  impressed  E.M.F.  is  due 
solely  to  the  electromotive  force  of  self-induction  and  may 


be  represented  by  — 
J\. 


whence 


Therefore 


rfT0' 
a 


By  integrating  from   the  initial   conditions  /  =  o,  /  =  — '  to 

J\. 

any  condition   /=/,/  =  /',   the  instantaneous  value  of   the 
current  is  found  to  be 

77  Rt 

r-f,-f, 

which  is  the  term  that  had  to  be  subtracted  in  the  formula 

for  the  growth  of  current. 
This  shows  clearly  that,  while 
self-induction  prevents  the  in- 
stantaneous attainment  of  the 
ultimate  value  of  the  current, 
there  is  eventually  no  loss  of 
energy,  since  what  is  sub- 
tracted from  the  growing  cur- 
rent is  given  back  to  the  decaying  current. 

Fig.  8  is  the  curve  of  decay  of  current  in  the  same  cir- 


DECAYING  CURRENT 
E.M.F.=  0 


MAGNETIC    LAWS    AND    FACTS.  2Q 

cuit  as  was  considered  in  Fig.  7.  The  ordinates  of  the 
one  figure  are  seen  to  be  complementary  to  those  of  the 
other. 

^20.  Quantity  of  Electricity  Traversing  a  Circuit  Due  to 
a  Change  of  Flux  Linked  with  it.  —  In  many  dynamo 
tests,  and  in  many  magnetic  investigations,  it  is  necessary 
to  measure,  generally  by  means  of  a  ballistic  galvanometer, 
the  quantity  of  electricity  traversing  a  circuit  due  to  a 
change  of  flux  linked  with  it.  If  the  circuit  have  a  resist- 
ance of  r  and  in  dt  time  the  flux  linked  with  n  turns 
changes  by  d$,  then  the  instantaneous  current 


i  = 


But  the  quantity  of  electricity  flowing  during  the  time  dt  is 
dq  =  idt,  hence 


which  is  independent  of  time.     So  if  the  flux  change  from 
<J>t  to  <I>2,  then 


* 


If  the  resistance  of  the  circuit  be  expressed  in  ohms  and 
the  flux  in  maxwells,  the  quantity  of  electricity  in  micro- 
coulombs  will  be 

Q 


ioo         R 

21.  Work  Performed  by  a  Conductor  Carrying  a  Current 
and  Moving  in  a  Magnetic  Field.  —  Let  a  conductor  carry- 
ing a  constant  current  i  be  moved  in  a  direction  perpen- 


3Q  DYNAMO    ELECTRIC   MACHINERY. 

dicular  to  itself  and  to  the  lines  of  force  of  a  magnetic  field. 
Suppose  it  to  move  for  dt  seconds,  and  in  that  time  to  cut 
d$  lines  of  force.  Then  the  induced  E.M.F.  e  will  be 

.     The  quantity  of  electricity  dq  that  has  to  traverse 

the  circuit  against  this  E.M.F.  during  the  time  dt  will 
be  idt.  Since  potential  is  a  measure  of  work,  the  work 
required  to  carry  dq  units  of  electricity  against  a  difference 
of  potential  e  is  edq  ergs.  Hence  the  work  in  ergs, 

d& 

dw  =  edq  =  idt  X  — —  =  id$. 
dt 

Therefore  the  current  i,  in  cutting  <I>  lines  of  force,  per- 
forms the  work 

w  =  /4>  ergs. 

From  this  it  is  seen  that  the  work  done  by  a  conductor 
carrying  a  current  and  cutting  lines  of  force  is  independent 
of  the  time  it  takes  to  cut  them. 

In  the  above  discussion,  if  the  field  be  non-uniform  or 
the  motion  be  irregular,  the  value  of  e  will  not  be  the 
same  for  each  instant  of  time.  But  since  the  result 
obtained  is  independent  of  time,  it  is  immaterial  how  the 
lines  are  arranged,  and  how  the  rate  of  cutting  varies. 

22.  Force  Exerted  between  a  Field  and  a  Conductor 
Carrying  a  Current.  —  When  a  conductor  moves  in  a  field 
perpendicular  to  itself  and  to  the  lines  of  force,  then,  from 
the  foregoing  article,  the  work  performed  is 

w  =  i&  ergs. 

If  the  conductor  be  /  centimeters  long,  and  traverses  a 
distance  of  d  centimeters  through  a  uniform  field  having  a 


MAGNETIC   LAWS   AND    FACTS.  31 

flux  density  of  (B  gausses,  then  the  total  number  of  lines 
of  force  cut  is 


and  the 

Work  =  ildfa  ergs. 

But 

Work  =  force  X  distance  =  Fd. 


, 

.  \  F  =  ^l(&  =  -  dynes. 
10 

This  force  acts  perpendicularly  to  the  wire  and  to  the  lines 
of  force. 

23.  Magnetomotive  Force  of  a  Circular  Circuit  Carrying 
a  Current.  —  A  thin  circular  conductor  carrying  a  current 
forms  a  magnetic  shell.  The  current  produces  magnetic 
lines  of  force  each  of  which  is  closed  upon  itself  and  is 
linked  with  the  conducting  circuit.  If  a  unit  magnet  pole 
be  carried  along  any  closed  path  linked  with  this  conductor, 
coming  back  to  point  of  starting,  it  will  be  subjected  to 
magnetic  forces  that  will  vary  in  magnitude  as  its  position 
is  changed.  These  forces  may  also  not  have  the  same 
direction  as  the  path  of  movement  at  any  instant.  But 
the  sum  of  the  products  of  the  lengths  of  the  path  elements 
into  the  corresponding  portions  of  the  force,  resolved  along 
the  path,  will  "represent  the  difference  of  magnetic  potential 
set  up  by  the  magnetic  shell.  This  sum  is  the  same  as 
the  product  of  the  average  force  by  the  length  of  the  path 
or  the  total  work  performed  upon  the  unit  pole. 

It  is  immaterial  whether  the  pole  be  carried  from  one 
side  of  the  shell  to  the  other,  Fig.  9,  or  the  shell  be  turned 
bottom  side  up  around  the  pole.  In  the  latter  case  it  is 


32  DYNAMO    ELECTRIC    MACHINERY. 

clear  that  all  the  lines  emanating  from  the  pole  will  be  cut 
once,  and  once  only,  by  the  conductor,  wherefore  4  n  lines 

will  have  been  cut. 

-<•»**  ~       "*"""*•. 

If  current  i  flows  in  the  con- 
ductor, then,  by  §  21,  the  work 
in  ergs  is 


If  there  be  n  turns  of  the  con- 
Fig.  9. 

ductor,  each  line  of  force  will  be 

cut  n  times,  and  the  work  will  be  4  nni  ergs.  Hence  the 
magnetic  difference  of  potential  between  the  two  sides 

of  a  thin  magnetic  shell  is  4  -nni  or  4  ™    •     Since  —  = 

10  10 

1.257,  and  since  the  current  and  the  number  of  turns  are 
usually  regarded  as  a  single  quantity,  namely  ampere-turns, 
it  follows  that  the  difference  of  magnetic  potential  is  equal 
to  1.257  times  the  number  of  ampere-turns. 

Difference  of  magnetic  potential  is  a  measure  of  the 
ability  of  the  shell  to  set  up  lines  of  force,  or,  in  other 
words,  is  a  measure  of  its  magnetomotive  force.  Magneto- 
motive force  and  difference  of  magnetic  potential  are  simi- 
lar, just  as  are  electromotive  force  and  difference  of  electrical 
potential.  Therefore  magnetomotive  force  may  be  ex- 
pressed as 

M.M.F.  =  1.257  nl. 

The  practical  unit  of  M.M.F.  is  the  gilbert,  so  that  a  gil- 
bert =  1.257  X  ampere-turn. 

24.  The  Toroid.  —  A  uniform  toroidal  winding  upon  a 
ring-shaped  iron  core  and  carrying  a  current  i  produces  the 
same  flux  density  at  all  points  on  the  axis  of  the  toroid. 
Assuming  the  core  to  be  removed  and  the  portion  of  the 


MAGNETIC  LAWS   AND    FACTS.  33 

flux  which  is  not  due  to  the  iron  to  have  the  same  distribu- 
tion as  the  flux  occasioned  by  iron,  then,  if  there  be  n  turns 
in  the  coil,  and  the  length  of  the  axis  be  /  cm.,  the  work 
necessary  to  be  exerted  upon  a  unit  pole  to  carry  it  once 
along  the  axis  would  be  4  iini  ergs,  which  is  also  equal  to 
the  product  of  field  intensity  3C  at  the  axis  into  its  length  /. 
Hence  the  magnetizing  force,  that  is,  the  strength  or  inten- 
sity of  field,  in  the  toroid  is 


lOl 

25.  Magnetization  Curves.  —  The  permeability  of  air  is 
constant  for  all  magnetizing  forces  ;  but  this  is  not  true  of 
iron  and  other  substances  having  permeabilities  noticeably 
greater  than  unity.  The  values  of  /*  for  these  paramag- 
netic substances  depend  also  upon  the  chemical  composition 
and  physical  condition  of  the  latter.  Values  of  magnetizing 
force,  3C,  and  the  corresponding  flux  density,  (B,  in  average 
commercial  wrought  iron,  in  cast  iron,  and  in  cast  steel  are 
given  in  the  following  table.  The  change  of  flux  density 
with  variation  of  magnetizing  force  is  best  shown  by  curves, 
called  magnetization,  or  (B-3C,  curves.  In  Figs.  10,  1  1,  and 
12  are  given  the  magnetization  curves  respectively  of 
wrought  and  sheet  iron,  cast  iron,  and  cast  steel,  the  curves 
being  plotted  from  the  tabulated  values.  From  an  inspection 
of  these  curves,  it  is  seen  that  an  increase  in  magnetizing 
force  results  at  first  in  a  marked  increase  of  flux  per  unit 
area,  but  as  the  material  becomes  magnetized  further  the 
increment  of  flux  density  lessens  and  finally  approaches  a 
value  proportional  to  the  corresponding  increment  of  mag- 
netizing force.  At  this  point  the  material  is  said  to  be 
sattirated,  that  is,  an  increase  of  M.M.F.  shows  no  increase 
of  flux  density  due  to  the  presence  of  the  given  material. 


34 


DYNAMO   ELECTRIC   MACHINERY. 


WROUGHT  AND  SHEET  IRON 


14000 

(B 

12000 


1LI  per  centimeter    16 


20         30        40        00        60 


80        90       100       110 


Permeability 


looo 


Fig.  10. 


?3,5 


CAST  IRON 


50         60         70         80         9C        100      110        120 


n  I  per  centimeter       16 
n I  per  inch 40 

"  Permeability/Z  100 


64  80 


Fig.  n. 


MAGNETIC   LAWS  AND    FACTS, 


35 


CAST  STEEL 


I90 

£    78 


i.05 


nl  per  centimeter 


•3C 


inch 


Fig.  12. 


DATA   FOR    (B-3C   CURVES. 

AVERAGE    FIRST-QUALITY    METAL. 


WROUGHT    AND 

CAST    IRON. 

CAST   STEEL. 

AMPERE- 

AMPERE- 

SHEET    IRON. 

3C 

PER    CEN- 

TURNS 
PER    INCH 

KILO- 

KILO- 

KILO- 

TIMETER 

LENGTH. 

/O 

LINES 

/n 

LINES 

/n 

LINES 

LENGTH. 

VJJ 

PER 

UJ 

PER 

05 

PER 

SQ.  IN  . 

SQ.    IN. 

SQ.  IN. 

10 

7-95 

2O.2 

1  1  8OO 

74 

3900 

25.2 

I2OOO 

77 

20 

15.90 

40.4 

I4OOO 

90 

5500 

35-5 

13800 

89 

30 

23^5 

60.6 

I52OO 

98 

6500 

42.0 

14600 

94 

40 

31.80 

80.8 

15800 

102 

7100 

45-7 

15400 

99 

5o 

39-75 

IOI.O 

16400 

106 

7700 

49-5 

16000 

103 

60 

47.70 

121.  2 

16800 

1  08 

8200 

53-o 

16400 

106 

80 

63-65 

l6l.6 

I72OO 

in 

8900 

57-2 

16700 

1  08 

TOO 

79.50 

2O2.O 

17600 

114 

9300 

60.0 

17600 

"3 

I25 

99.70 

252-5 

I78OO 

H5 

9700 

62.4 

l82OO 

117 

150 

119.25 

303-0 

I8OOO 

116 

IOIOO 

65.8 

18600 

120 

3C  =  1-257  O/per  cm.)  =  .495  («/per  in.).     (B  =  .155  (*  per  sq.  in.) 


36  DYNAMO   ELECTRIC   MACHINERY. 

Curves  of  permeability,  plotted  in  terms  of  flux  density, 
for  wrought  iron,  cast  iron,  and  cast  steel  are  also  shown 
in  the  accompanying  figures. 

In  general  all  substances  mixed  with  or  alloyed  with  iron 
lower  its  permeability.  In  steel  and  cast  iron  the  permea- 
bility seems  to  be  in  inverse  proportion  to  the  amount  of 
carbon  present.  Carbon  in  the  graphitic  (not  combined) 
form  lowers  the  permeability  less  than  carbon  when  com- 
bined. In  cast  iron  and  cast  steel  such  substances  as  tend 
to  give  softness  and  greater  homogeneity  to  the  metal 
when  present  in  limited  amounts,  say  2  per  cent,  increase 
the  value  of  /*.  Aluminum  and  silicon  act  in  this  way. 

The  physical  condition  of  the  metal  also  affects  its  per- 
meability. Chilling  in  the  mold,  when  casting,  lowers  it, 
as  does  tempering,  or  hardening  the  metal  by  working  it. 
On  the  other  hand,  annealing  increases  the  permeability. 

A  piece  of  iron  or  steel,  subjected  to  a  small  magnetizing 
force,  has  its  permeability  increased  by  increasing  the  tem- 
perature until  a  critical  temperattire  is  reached,  when  it 
falls  off  rapidly  to  almost  unity.  For  stronger  magnetiza- 
tion the  permeability  does  not  rise  so  high  at  the  critical 
temperature,  and  does  not  fall  off  so  sharply  after  it.  The 
value  of  this  critical  temperature  lies  between  650°  C.  and 
900°  C.,  depending  on  the  test  piece.  The  influence  of 
temperature  upon  permeability  is  very  small  for  the  changes 
in  temperature  occurring  in  practical  operation,  and  is 
therefore  negligible. 

26.  Reluctance  and  Permeance.  —  In  the  flow  of  mag- 
netic lines  of  force  the  reciprocal  of  the  permeability,  -,  is 

called   the  reluctivity.     The  total  reluctamet  tending   to 
oppose  the  passage  of  magnetic  lines  under  the  influence 


MAGNETIC   LAWS   AND    FACTS.  37 

of  a  magnetic  difference  of  potential,  is  directly  as  the 
length  and  the  reluctivity  of  the  medium  and  inversely  as 
its  cross-section.  Hence  the  total  magnetic  resistance  or 

reluctance  =  -  ^^  —  :  —  reluctivity. 
cross-section 

Reluctivity  is  usually  represented  by  y  t  =  —  ).      Hence  for 

a  medium  of  cross-section  A  square  centimeters  and  length 
/  centimeters,  the  reluctance 

(R=  -y. 
A 

The  unit  in  which  (R  is  expressed  is  called  the  oersted. 

Permeance  is  the  reciprocal  of  the  reluctance,  hence  the 
permeance 

A       A 


It  must  be  remembered  that  y  and  /*  are  not  constant  for 
some  substances,  but  depend  for  their  values  upon  the 
strength  of  the  magnetizing  force  3C  which  is  acting  upon 
the  substances. 

27.  Relation  Between  Magnetomotive  Force,  Magnetic 
Flux,  and  Reluctance.  —  The  flux  produced  in  a  magnetic 
circuit  by  a  magnetomotive  force  may  be  expressed  by  an 
equation  similar  to  that  expressing  the  current  flow  in  an 
electric  circuit  due  to  an  impressed  electromotive  force, 
which  is 

electromotive  force 
current  =  -  :  -  • 
resistance 

The  corresponding  equation  for  the  magnetic  circuit  is 

.    n  magnetomotive  force 

magnetic  flux  =  —  -  -  —       --  , 
reluctance 


38  DYNAMO   ELECTRIC   MACHINERY. 

or  symbolically  / 

M.M.F.       4  ™  m 
<f>  =  -  =  -- 

(R  l 


Since  the  unit  of  magnetic  flux  is  one  line  of  force  or  the 
maxwell,  the  unit  of  magnetomotive  force  is  the  gilbert, 
and  the  unit  of  reluctance  is  the  oersted,  this  equation  may 
be  written 

,,        gilberts 
maxwells  =  -          —  • 
oersteds 

The  application  of  this  equation  is  not  as  simple  as 
that  of  the  corresponding  equation  of  the  electric  circuit. 
Electric  circuits,  in  general,  exist  in  media  of  zero  electric 
conductivity,  and  therefore  permit  of  accurate  calculations, 
since  the  leakage  is  inappreciable.  Magnetic  circuits,  on 
the  other  hand,  are  situated  in  media  which  have  permea- 
bilities of  at  least  unity  and  hence  much  leakage  is  present, 
and  precise  calculations  require  a  consideration  of  all  flux 
paths.  In  the  designing  of  dynamo  electric  machinery, 
however,  one  or  more  paths  of  low  reluctance  are  presented 
to  the  magnetizing  force,  and  these  are  so  shaped  that  the 
leakage  paths  offer  a  comparatively  high  reluctance. 

28.  Hysteresis.  —  If  a  piece  of  iron  become  magnetized, 
and  the  magnetizing  force  be  then  removed,  the  iron  does 
not  become  completely  demagnetized.  A  certain  magnet- 
izing force  in  the  opposite  direction  must  be  applied  to 
bring  it  back  to  its  original  condition.  This  phenomenon, 
where  "  changes  of  magnetism  lag  behind  the  changes  of 
force,"  has  been  termed  hysteresis.  Because  of  hysteresis 
a  (B-3C  curve  taken  with  continuously  increasing  values  of  3C 
to  the  maximum  and  then  with  continuously  decreasing 


MAGNETIC    LAWS   AND   FACTS. 


39 


values  of  3C  to  a  negative  maximum,  and  so  on,  will  assume 
the  shape  shown  in  Fig.  13.     The  distance  OA  represents 


AGNETIZING  /  Q 


Fig.  13. 

the  coercivity,  that  is,  the  magnetizing  force  necessary  to 
bring  the  iron  from  a  magnetic  to  a  neutral  state.  The 
distance  OC  represents  the  retentivity,  that  is,  the  value 
of  the  residual  magnetic  flux  density  in  the  iron  after  the 
magnetizing  force  has  been  removed. 

The  area  inclosed  by  the  curve  represents  the  energy 
lost  in  carrying  the  iron  through  one  cycle,  i.e.  from  a 
maximum  magnetization  in  one  direction  to  a  maximum  in 
the  opposite  direction  and  back  to  the  original  condition. 
Suppose  the  magnetization  to  be  due  to  a  current  7  flowing 
in  a  solenoid  of  n  turns.  If,  in  a  short  interval  of  time  dt, 


40  DYNAMO   ELECTRIC   MACHINERY. 

a  change  of  d&  be  made  in  the  flux  which  is  linked  with 
the  solenoid,  then  this  change  will  induce  an  E.M.F.  in  the 
solenoid,  which,  during  the  interval  of  time  dt,  will  be 
equal  to 


E=-^j-  volts. 
io*dt 

During  this  time  work  must  be  performed  to  maintain  this 
current  /,  and  its  magnitude  is 

Eldt** 


io8 

for  Idt  represents  the  quantity  of  electricity  which  is  trans- 
ferred from  one  point  to  another,  between  which  there  ex- 
ists a  difference  of  potential  E.  Now  <£  =A(&  (§  14)  and 

T  O  fT*7 

hence  d3>  =  Ad($>.  Furthermore,  nl  =  -  -  (§  24).  Hence 
the  work  during  the  time  dt  is 

Al 
Eldt  =  — —  SCiftg  joules. 

Supposing  the  magnetizing  force  to  vary  cyclically,  taking 
t  seconds  to  make  one  cycle,  then  the  work  per  cycle  is 


Elt 


p-\-($>m 

'•    I  JCdCB  joules. 
nJ  J 


If  the  number  of  cycles  completed  in  one  second  be/,  then 
/  =  -,  and  the 
in  watts,  equals 


/  =  -,  and  the  power  in  joules  per  second,  that  is,  the  power 


+(B 


fAi   rm    re-7     rm 

El  =  -4^-    /  3C^«  =  —  fv  I  3Cd(B, 
I0747rj  _(Bm       47r       J    _ftm 

where  v  is  the  volume  of  iron  in  cubic  centimeters.     The 


MAGNETIC    LAWS  AND    FACTS.  41 

integral  expression  is  evidently  the  area  contained  by  the 
hysteresis  loop. 

The  value  of  this  integral  is  dependent  upon  (Bm,  upon 
the  retentivity  of  the  kind  of  iron,  and  upon  its  coercivity. 
Steinmetz  has  shown  that  for  all  practical  purposes  the 
value  of  the  integral  for  flux  densities  ranging  from  2000 
to  14000  gausses  may  be  expressed  by  the  empirical  formula 


T    />  +  ®m 
—  /  Xd<& 

J 


where  y  is  a  constant  depending  upon  the  physical  and 
chemical  properties  of  the  iron.  Therefore  the  power  lost 
in  watts  due  to  hysteresis  may  be  written 


Values  of  the  constant  7?  are  given  in  the  following  table  :  — 

HYSTERETIC    CONSTANTS. 

Best  soft  iron  or  steel  sheets     .........  o.ooi 

Good  soft  iron  sheets       ...........  0.002 

Ordinary  soft  iron       ............  0.003 

Soft  annealed  cast  steel        ..........  0.008 

Cast  steel       ...............  0.012 

Cast  iron  ................  0.016 

Hard  cast  steel  ..............  0.025 

The  hysteretic  constant  increases  with  continued  heating, 
and  this  effect  is  called  ageing.  Annealing,  while  it  in- 
creases the  permeability,  also  increases  the  hysteretic  con- 
stant as  well  as  the  ageing  effect. 

The  magnitude  of  the  hysteretic  constant  is  largely  de- 
pendent upon  the  mechanical  structure  of  the  iron.  To 
attain  the  smallest  value,  the  iron  should  not  be  of  homoge- 
neous structure,  but  should  be  more  compact  in  directions 
perpendicular  to  the  direction  of  the  flux  than  in  transverse 
directions 


42  DYNAMO   ELECTRIC   MACHINERY. 

^29.  Eddy  Currents.  —  When  a  mass  of  iron  is  subjected 
to  a  pulsating  flux,  electromotive  forces  are  set  up  in  the 
iron  which  produce  currents  therein,  called  eddy,  or  Fou- 
cault,  currents.  The  flow  of  these  currents  represents  an 
expenditure  of  energy  appearing  as  heat.  In  order  to 
prevent  excessive  heating  of  such  portions  of  dynamo 
electric  machinery  subject  to  rapid  reversals  or  changes  of 
magnetization,  these  portions  are  constructed  of  laminated 
iron,  the  laminae  being  transverse  to  the  direction  of  flow 
of  the  eddy  currents,  but  longitudinal  with  the  magnetic 
flux.  Each  lamina  is  more  or  less  thoroughly  insulated 
from  its  neighbors  by  the  natural  oxide  on  the  surface  or 
by  Japan  lacquer. 

The  loss  of  power  due  to  eddy  currents  could  be  made 
inappreciable  by  the  use  of  laminae  sufficiently  thin ;  but  a 
limitation  exists  due  to  the  decrease  in  effective  iron  cross- 
section  caused  by  the  waste  of  space  which  is  taken  up  by 
the  insulation  between  adjacent  laminae.  The  thickness 
of  the  laminae  generally  used  for  dynamo  armatures  is 
between  0.014  and  0.02  inch,  and  the  space  between 
laminae  is  usually  somewhat  less  than  0.002  inch. 

A  formula  for  the  calculation  of  the  power,  in  watts, 
lost  in  iron  due  to  eddy  currents,  based  upon  the  assump- 
tion that  the  laminae  are  perfectly  insulated  from  each 
other,  is 

Pe  =  kvf2l2&2m, 

where     k  =  a  constant  depending  upon  the  resistivity  of 

the  iron,  its  value  being  about  1.6  X  io~u, 
v  =  volume  of  iron  in  cubic  centimeters, 
/  =  thickness  of  one  lamina  in  centimeters, 
/  =  number  of  magnetic  cycles  per  second, 

and      (&m  =  maximum  flux  density  (i.e.  <J>m  per  sq.  cm.). 


PROBLEMS.  43 

The  armatures  of  dynamos  are  usually  provided  with 
projecting  teeth,  and  therefore  the  flux  density  between 
an  armature  and  its  field  poles  is  greatest  opposite  the 
teeth  and  is  a  minimum  opposite  the  slots.  As  the  arma- 
ture rotates,  this  variation  of  flux  produces  to  some  extent 
eddy  currents  in  the  pole  faces.  To  reduce  the  loss  occa- 
sioned thereby,  the  pole  faces  are  sometimes  also  constructed 
of  laminated  iron. 

PROBLEMS. 

1.  Two  cylindrical  magnets,  1.8  cm.  in  diameter,  are  mag- 
netized to  an  intensity  of  500  units  pole  for  each  square  cm.  of 
cross-sectional  area,  and  their  north  poles  are  placed  8  cm. 
apart.     Compute  the  force  of  repulsion  between  the  two  north 
poles. 

2.  What  is  the  total  flux  from  each  pole  of  the  magnets 
specified  in  Prob.  i,  considering  the  poles  to  be  isolated  and 
concentrated  at  points  ? 

3.  A  conductor  24  inches  long,  moving  parallel  to  itself  and 
at  right  angles  to  a  magnetic  field  having  an  intensity  of  40000 
maxwells  per  sq.  in.,  traverses  5  ft.  in  3.5  seconds.     Determine 
the  average  E.M.F.  in  volts  induced  in  the  conductor  during 
this  interval. 

4.  In  gV  second  the  current  strength  in  a  circuit,  having  an 
inductance  of  0.6  henry,  falls  from  30  to  15  amperes.     What  is 
the  magnitude  and  direction  of  the  average  induced  E.M.F.  in 
the  circuit  due  to  this  change  of  current  ? 

5.  What  is  the  inductance  of  a  circuit  having  5  ohms  resist- 
ance and  in  which  the  instantaneous  value  of  the  current  0.03 
second  after  impressing   no  volts   upon   the    circuit  is    13.9 
amperes  ? 

6.  What  would  be  the  current  in  a  circuit,  having  10  ohms 
resistance  and  0.3  henry  inductance,  0.02  second  after  suppress- 
ing the  initial  E.M.F.  of  80  volts  ? 


44  DYNAMO   ELECTRIC    MACHINERY. 

7.  The  force  exerted  on  a  wire  5  ft.  long,  which  carries  a 
current  of  50  amperes,  is  1800  dynes.     What  is  the  intensity  of 
the  magnetic  field  in  which  this  wire  is  situated  ? 

8.  A  brass  toroid,  having  a  mean  diameter  of  20  cm.,  is 
completely  wound  with  8  turns  of  wire  per  cm.  of  axial  length. 
Determine  the  magnetic  field  intensity  at  the  axis,  when  a  cur- 
rent of  50  amperes  flows  through  the  winding.     Calculate  the 
total  magnetomotive  force  which  sets  up  this  field. 

9.  A  wrought-iron  toroid,  wound  with  20  turns  of  wire  per 
inch  of    axial   length,   has  a  current  of   3.5    amperes    flowing 
through  the  winding.     Determine  the  flux  density  and  perme- 
ability of  the  iron  from  the  data  given  in  §  25. 

10.  The  flux  density  in  a  cylindrical  cast-iron  rod  5  cm.  in 
diameter  and  30  cm.  long  is  6000  gausses  (/z=  250).    Compute 
the  reluctance  and  permeance  of  the  rod  between  the  two  faces. 

1 1.  Calculate  the  total  number  of  ampere-turns  necessary  to 
produce  a  flux  density  of  6000  gausses  in  the  toroid  of  Prob.  10. 

12.  A   closed   core,    composed   of    the   best    steel   sheets 
0.035  cm-  thick,  has  a  volume  of  5400  cu.  cm.,  and  is  subjected 
to  loo  magnetic  reversals  per  second,  i.e.  f  =•  50.      Calculate 
the  hysteresis  and  eddy  current  losses  when  the  maximum  flux 
density  in  the  core  is  3500  gausses. 


ARMATURES.  45 


CHAPTER    III. 

ARMATURES. 

30.  Dynamos.  —  Dynamos  may  be  defined  as  machines 
to  convert  mechanical  energy  into  electrical  energy,   or 
electrical  energy  into  mechanical  energy,  by  utilizing  the 
principle  of  electromagnetic  induction.     A  dynamo  is  known 
as  a  generator  when  mechanical  energy,  supplied  in  the 
form  of  rotation,  in  all  commercial  machines,  is  converted 
into  electrical  energy,  which  may  be  delivered  either  as 
" direct  current"  or  as  "alternating  current."     When  the 
conversion  of  energy  takes  place  in  the  reverse  order,  the 
dynamo  is  called  a  motor. 

31.  Principle  of  Action  of  a  Generator.  —  If  a  loop  of 
wire  be  revolved  in  a  magnetic  field  about  an  axis  perpen- 
dicular to  the  lines  of  force,  as  in  Fig.    14,  then  each  side 
(but  not  the  ends)  of  the  loop  is  a  conductor  moving  across 
the  lines  of  a  magnetic  field,  and  as   such   will  have  an 
E.M.F.  induced  in  it.     Since  the  motion  of  one  conductor 
is  up  while  that  of  the  other  is  down,  the  directions  of  the 
induced  E.M.F.'s  in  the  two  sides  would  be  opposite  to 
each  other,  but  since  they  are  on  opposite  sides  of  a  loop, 
the  pressure  will  be  cumulative;  i.  e.  instead  of  neutralizing 
each  other,  the  two  pressures  will  be  added  to  each  other. 
If  now  the  two  ends  of  the  wire  from  which  the  loop  is 
made  be  respectively  connected  with  slip  rings,  and  a  cir- 
cuit be  completed  through  contacts  sliding  on  them,  a  cur- 
rent will  flow.     When  the  loop,  in  its  revolution,  reaches  a 


46  DYNAMO   ELECTRIC   MACHINERY. 

position  (as  illustrated  in  Fig.  14)  such  that  the  conductor 
that  was  previously  moving  upward  begins  to  move  down- 
ward, then  the  direction  of  the  induced  E.M.F.  will  be 
changed  in  both  sides  of  the  loop,  and  the  direction  of  the 

\ 


Fig.  14. 

current  through  the  circuit  will  be  changed.  For  each 
complete  revolution  the  current  changes  direction  twice. 
It  is  an  alternating  current,  and  the  supposed  machine  is 
an  alternating-current  generator,  or  simply  an  alternator. 

32.  The  Function  of  the  Commutator.  —  If,  instead  of 
connecting  the  two  ends  of  a  loop  of  wire  revolving  in  a 
magnetic  field  to  slip  rings,  they  be  attached  one  to  each 
half  of  a  split  metal  ring  mounted  on  the  same  shaft,  the 
two  halves  being  insulated  from  each  other,  and  brushes  be 
provided,  which  are  so  placed  that  at  the  instant  the  induced 
E.M.F.  in  the  loop  changes  in  direction  the  brushes  will 
slide  across  from  one  of  the  halves  to  the  other,  then  the 
current,  while  reversed  in  the  loop,  will  flow  in  the  same 
direction  in  the  external  circuit.  This  arrangement,  called 


ARMATURES. 


47 


a  commutator,  is  employed  when  it  is  desired  to  obtain  a  recti- 
fied, continuous  or  direct  current.  A  dynamo  so  equipped 
is  called  a  direct-current  generator,  or  simply  a  generator. 

If  the  loop  were  wound  double,  i.e.  have  four  conductors, 
before  the  ends  were  attached  to  commutator  segments, 
and  if  the  speed  and  the  strength  of  the  magnetic  field  be 
maintained  constant,  twice  the  E.M.F.  will  be  produced. 

For  a  single  loop,  the  commutator  would  consist  of  two 
cylindrical  pieces  or  segments,  as  shown  in  Fig.  15.  In 
this  case  there  would  be  no 
E.M.F.  produced  at  the  in- 
stants when  the  brushes  pass 
from  one  segment  to  the 
other,  and  hence  the  current 
would  fall  to  zero  twice  during 
every  revolution  of  the  loop. 
If  two  loops,  placed  at  right  Fig-  I5' 

angles  to  each  other,  are  rotated  in  a  magnetic  field,  one 
or  the  other  would  always  be  cutting  lines  of  force  and  at 
no  time  could  the  pressure  be  zero.  To  satisfactorily  collect 

current  from  this  arrange- 
ment requires  four  commu- 
tator segments  and  a  system 
of  connections  similar  to  that 
25*  shown  in  Fig.  16.  In  this 
case  the  E.M.F.  would  fluc- 
tuate, but  not  so  badly  as  in 
the  previous  one.  If  the  num- 
ber of  loops  be  increased  and 
Fig.  16.  the  number  of  commutator 

segments  be  correspondingly  increased,  the  E.M.F.  fluctua- 
tion of  such  an  arrangement  will  become  practically  negligible. 


48  DYNAMO   ELECTRIC   MACHINERY. 

33.  Electromotive  Force  Generated.  —  The  magnitude  of 
the  electromotive  force  induced  in  a  conductor  of  length 
/  cm.  moving  parallel  to  itself  with  a  velocity  of  v  cm.  per 
sec.  across  a  uniform  magnetic  field  having  a  flux  density  of 
(B  gausses  is 

E  =  (Rlv  io~8  sin  a  volts,  §  15 

where  a.  is  the  angle  between  the  paths  of  the  flux  and  of 
the  conductor.  If  a  single  loop  (two  conductors)  revolve 
about  an  axis  that  is  in  the  plane  of  the  loop  and  perpen- 
dicular to  the  flux  with  an  angular  velocity  of  ^revolutions 
per  minute,  the  linear  velocity  of  the  conductors  will  be 

v  =  2  nr  —t 
60 

where  r  is  the  distance  in  cm.  of  each  conductor  from  the 
axis.  The  instantaneous  E.M.F.  for  a  loop  of  s  conductors 
is  therefore 

v 

E'  =  2  Ti&lrs  •   —  IO~8  sin  a  volts. 
60 

But  2  rl®>  is  the  maximum  flux  parsing  through  -the  loop, 
®m  ;  hence 

E'  =  n$ms-r-  io~8  sin  a  volts.  (i) 

oo 

The  maximum  value  of  the  induced  E.M.F.  for  the  loop 
of  s  conductors  is  attained  when  a  =  90°,  i.e.  when  the 
conductors  move  perpendicularly  across  the  magnetic  flux. 
This  maximum  value  is 


(2) 


The  instantaneous  E.M.F.  is  therefore 
E'  =  Em  sin  «, 


ARMATURES. 


49 


which  is  the  equation  of  a  sine  curve.  Thus  the  sine  curve, 
Fig.  17,  shows  the  instantaneous  values  of  the  induced  elec- 
tromotive force  as  the  angle  a  varies  from  o°  to  360°. 


27T 


90 


60 


Fig.  17. 


The  average  E.M.F.  during  a  half  revolution  is  obtained 
by  dividing  the  area  of  one  lobe  by  the  base  line.     Thus 

„  sin  ad  a 


Substituting   the  value   of   Em,  there  results  the  average 
E.M.F.  induced  in  the  loop  of  s  conductors 

V 


io~8  volts. 


(4) 


If  this  loop  be  provided  with  a  commutator  as  explained 
in  the  foregoing  section, 
the  direction  of  the  vol- 
tage impressed  on  the 
external  circuit  remains 
the  same,  but  the  magni- 
tude varies  sinusoidally 


90 


270 


360 


as  shown  in  Fig.  18. 

If  a   similar   loop  be 
placed  90°  from  the  other,  the  magnitude  of  the  induced 


180 

a 
Fig.  18. 


37T 


Fig.  19. 


50  DYNAMO   ELECTRIC    MACHINERY. 

E.  M.  F.   therein  will  be  the  same  as  that  in  the  first,  but 
corresponding  instantaneous  values  in  the  two  loops  occur 

90°  apart,  as  shown  in 
Fig.  19.  The  resulting 
electromotive  force  at  any 
instant  may  be  found  by 
adding  the  E.M.F.'s  in- 
duced in  the  two  loops  at 
that  instant.  Thus  in 
Fig.  19  the  dotted  line 
shows  the  resulting  pres- 
sure for  two  loops  situated  90°  apart. 

The  instantaneous  value  of  the  electromotive  force  in- 
duced in  the  first  loops  is 

E/  =  Em  sin  «, 
and  that  induced  in  the  second  is 

E2'=Emsm  (90°  + a); 

therefore  the  resulting  instantaneous  value  of  voltage  in- 
duced in  the  loops  when  connected  in  series  is 

E'  =  E/  +  E/  =  Em  [sin  a  +  cos  «]. 

In  general,  if  there  be  m  loops  connected  in  series  and 

displaced  —  electrical  degrees  from  each  other,  and  rotating 
m 

about  a  common  axis  in  a  uniform  magnetic  field,  then 
the  instantaneous  pressure  will  be 


'  =  EJsina  +  sinf--  +  a  J+  .  .  +  sin  In  —    -+aj  ,  (5) 


ARMATURES.  51 

and  the  average  pressure  will  be 


=Jk  f 
_7r_J0 
2  m 

+  sin  (  TT  —  --  ha  )  Ida  ; 
\        m  /J 


whence 

JE     =       m   m/  =  2  m$ms  •  — -io~8  volts.  (6) 

TT  oo 

The  total  number  of  conductors  S,  connected  in  series 
and  in  circuit  between  two  brushes,  is  equal  to  the  product 
of  the  number  of  loops  m  and  the  number  of  conductors 
per  loop,  s ;  or 

51  =  ms. 

By  substituting  this  value  in  (6),  there  is  between  brushes 

(7) 

w 

The  maximum  and  minimum  values  of  instantaneous  pres- 
sures are  obtained  respectively  by  substituting  for  a  in  (5), 

the  quantities  —  and  o.     The  resulting  expressions  are 
2  m 


=   Em  CSC 


and 


where  Em  is  the  maximum  E.M.F.  per  loop.     The  percent- 


52  DYNAMO   ELECTRIC   MACHINERY. 

age  fluctuation  of  the  E.M.F.,  therefore,  can  be  represented 
by 


I0o  X     max  ~     m[n      or  csc  —  --  cot  —   •      (8) 

E&v  m  L     '2m  2  m  J 

Thus,  for  twelve  loops  revolving  in  a  bipolar  field,  the 
fluctuation  of  electromotive  force  is  1.7  per  cent. 

In  order  to  render  the  foregoing  formulae  applicable 
to  a  multipolar  field,  it  is  necessary  to  insert  the  symbol 
/,  which  denotes  the  number  of  pairs,  of  field  poles.  The 

product  of  the  terms  —  and  /  represents  the  number  of 
60 

magnetic  cycles  some  of  the  iron  of  the  machine  passes 
through  in  one  second.  It  is  termed  frequency,  as  in 
alternating-current  work,  and  is  represented  by/.  Thus 


Therefore  the  average  electromotive  force  available  between 
brushes  is 

£av  =  2<!>mSfio-»  volts,  (9) 

where  4>OT  is  the  total  flux  per  pole  passing  through  the 
armature. 

34.  The  Armature.  —  In  a  dynamo,  the  loops  of  wire  in 
which  E.M.F.  is  induced  by  movement  in  a  magnetic  field, 
together  with  the  iron  core  that  sustains  them,  with  the 
necessary  insulation,  and  with  the  parts  connected  imme- 
diately thereto,  constitute  the  armature  of  a  dynamo.  An 
armature  in  which  both  sides  of  the  loop  of  wire  cut  lines 
of  force,  as  in  the  cases  just  described,  is  called  a  Drum 
Armature.  A  kind  of  armature  less  generally  used  is  the 


ARMATURES.  53 

Ring  Armature,  illustrated  diagrammatically  in  Fig.  20. 
Here  the  lines  of  force  emanating  from  the  N.  pole  flow 
through  the  iron  core  of 
the  ring,  and  very  few 
across  the  air  space  inside 
the  ring.  Hence,  when 
wires  are  wound  on  the 
ring,  and  the  whole  is 
revolved  about  an  axis  per- 
pendicular to  the  plane  of 

the  ring,  only  the  wires  on  the  outside  face  of  the  ring 
cut  lines  of  force,  those  on  the  inside  serving  only  to  com- 
plete the  electrical  circuit. 

Drum  armatures  have  all  the  conductors  on  the  peripheral 
surface,  and  therefore  have  a  greater  percentage  of  active 
wire  than  ring  armatures.  Drum  armature  cores  are 
very  often  constructed  in  the  form  of  a  ring,  because 
of  better  ventilation  and  economy  of  iron ;  and  such 
construction  should  not  be  confused  with  that  of  ring 
armatures. 

A  drum  armature  of  large  diameter  and  of  short  length 
in  the  axial  direction  may  have  more  wire  exposed  on  its 
ends  than  on  its  periphery.  The  pole  pieces  are  some- 
times placed  at  the  ends,  and  the  armature  is  then  called 
a  Disk  Armature.  This  type  is  seldom  used  in  this 
country. 

35.  The  Field  Magnets.  —  Almost  all  dynamos  have  their 
magnetic  fields  produced  by  electro-magnets,  and  these  are 
called  field  magnets.  In  small  machines  the  field  magnets 
are  usually  bipolar,  i.e.  they  have  one  North  and  one  South 
pole,  with  the  armature  revolving  between  them.  Bipolar 
machines  are  made  in  many  forms,  a  few  of  which  are 


54 


DYNAMO   ELECTRIC   MACHINERY. 


shown  in  Figs.  21,  22,  and  23.     The  magnetizing  coils,  or 
field  coils,  may  be  placed  on  both  legs  of  the  magnet,  on 


Fig.  21. 


Fig.  22 


one  leg,  or  on  the  yoke  which  connects  the  two.     The  best 
and  most  used  bipolar  arrangement  is  the  enclosed  type  of 


Fig.  23. 


Fig.  23,  for  the  coils  are  almost  completely  surrounded  by 
iron,  thereby  securing  protection  from  mechanical  injury, 
as  well  as  avoiding  excessive  magnetic  leakage. 


ARMATURES. 


55 


A  typical  form  of  multipolar  field  magnet  structure  is 
shown  in  Fig.  24,  in  which  an  even  number  of  poles  are  so 
wound  as  to  be  alternately  magnetized  North  and  South. 


Fig.  24. 

36.  Armature  Windings.  —  It  is  possible  to  connect  the 
conductors  of  an  armature  to  each  other  and  to  the  com- 
mutator segments  in  a  great  many  ways  that  will  permit  of 
satisfactory  operation.  The  design  of  an  operative  scheme 
of  winding  is,  to  a  great  extent,  a  geometrical  problem.  Of 
the  many  possible  and  proposed  schemes  of  winding,  those 
which  have  been  adopted  and  are  still  used  in  the  construc- 
tion of  standard  machines  are  characterized  by  economy 
of  copper,  by  good  time  constants  to  secure  satisfactory 
commutation,  and  by  such  coil  shapes  as  permit  of  con- 
venience in  construction  and  assembling  and  of  accessibility 
for  making  repairs. 

Direct-current  armature  windings  are  divided  into  two 


56  DYNAMO   ELECTRIC   MACHINERY. 

classes,  namely  open-coil  and  closed-coil  windings.  The 
former  are  used  almost  exclusively  on  series  constant-cur- 
rent machines,  such  as  the  Thomson-Houston  arc-light 
generators,  and  will  be  discussed  in  a  later  chapter.  With 
openrcoil  windings,  only  those  conductors  which  are  con- 
ductively  connected  between  the  commutator  segments, 
which  the  brushes  are  momentarily  resting  on,  are  effective 
in  supplying  an  electromotive  force. 

Closed-coil  windings  are  much  more  generally  used.  If 
the  wire  of  an  ordinary,  that  is,  single,  closed-coil  winding 
were  removed  from  the  armature  and  uncoiled,  it  would 
form  a  closed  loop,  and  the  points  of  connection  with  the 
commutator  segments  would  be  equidistant  from  each  other. 
Some  closed-coil  windings  are  so  constructed  that  the  wire, 
if  removed  from  the  armature  core  and  uncoiled,  would  form 
two  or  more  closed  endless  loops.  Such  windings  are 
termed  duplex,  triplex,  or  quadruple*  windings,  according 
to  the  number  of  endless  loops,  whether  two,  three,  or  four. 
Such  multiplex  windings  are  sometimes  employed  on  ma- 
chines of  large  current  output ;  but  their  use  is  relatively 
infrequent,  and  therefore,  unless  explicitly  stated  to  the 
contrary,  single,  or  simplex,  windings  will  hereinafter  be 
understood. 

It  is  convenient,  in  treating  of  armature  windings,  to  call 
each  of  the  portions  which  terminates  at  two  commutator 
segments  an  element  of  the  winding.  An  armature  element 
may  consist  of  one  or  more  armature  coils.  Those  parts 
of  an  armature  element  which  lie  on  the  periphery  of  the 
armature  and  in  which  E.M.F.'s  are  induced  are  called  in- 
ductors. Thus,  in  the  ring  type  of  armature,  only  the  por- 
tions of  the  wire  on  the  outer  surface  of  the  ring  constitute 
its  inductors.  A  drum  armature  element,  however,  has  two 


ARMATURES.  57 

inductors  lying  axially  on  the  core  surface.  In  the  follow- 
ing discussion  of  armature  windings,  when  one  inductor  is 
mentioned  it  does  not  imply  that  only  one  wire  is  meant ; 
further,  an  element  said  to  be  formed  by  two  inductors  may 
be  a  coil  of  many  turns.  Simplification  of  the  winding 
diagrams  is  effected  in  this  manner. 

Two  types  of  closed-coil  windings  for  direct -current  arma- 
tures are  to  be  distinguished,  namely  two- circuit  or  wave 
windings,  and  multiple-circuit  or  lap  windings;  the  former 
being  used  principally  on  machines  of  small  output,  and  the 
lap  winding  on  machines  of  intermediate  and  large  output. 
In  the  wave  winding  there  are  always  two  circuits  between 
the  brushes,  regardless  of  the  number  of  field  poles  on  the 
machine,  each  of  the  circuits  carrying  one-half  of  the  total 
current.  For  this  type  of  winding  only  two  brushes  are 
required,  but  it  is  usual  in  practice  to  provide  as  many 
brushes  as  there  are  poles,  in  order  to  avoid  excessive  spark- 
ing at  the  commutator.  There  is  a  slight,  but  generally 
neglected,  E.M.F.  induced  in  the  portions  of  the  winding 
included  at  any  instant  between  the  different  sets  of  brushes 
of  the  same  polarity.  It  is  quite  small,  however,  as  com- 
pared with  the  E.M.F.  induced  between  brushes  of  opposite 
polarity.  In  the  lap  winding  there  are  as  many  circuits 
between  the  brushes  as  there  are  field  poles,  and  each  cir- 
cuit carries  —  of  the  total  current.  For  armatures  of  this 
2p 

type  as  many  brushes  as  field  poles  are  required. 

Starting  from  a  certain  commutator  segment  and  passing 
clockwise  over  p  elements  of  a  simplex  wave  winding,  or 
one  element  of  a  simplex  lap  winding,  one  reaches  the  next 
adjacent  segment  either  to  the  right  or  to  the  left.  If  it  be 
to  the  right  it  is  called  a  progressive  winding ;  and  if  to  the 


DYNAMO   ELECTRIC   MACHINERY. 


left,  it  is  called  a  retrogressive  winding.     The  use  of  the 

latter  is  somewhat  more  economical  in  copper. 

A  4-pole  two- circuit,  or  wave  winding  is  shown  diagram- 

maticallyin  Fig.  25,  in  which  the  inductors  are  represented 

by  the  short  radial  lines 
and  the  end  connections  by 
the  lines  joining  them. 
The  brushes  are  placed 
inside  the  commutator  for 
clearness.  Fig.  26  shows 
the  same  winding  more 
clearly  in  developed  form, 
the  seventeen  commutator 
segments  being  lettered 
from  a  to  q.  Tracing  the 
Fig.  25.  method  of  interconnecting 

the  34  armature  inductors,  and  starting  from  segment  a, 


y 


Fig.  26. 

it  is  seen  that   connection   is  made  to  inductor  No.   10, 
which  is  connected  at  the  rear  to  inductor  No.   19 ;  and 


ARMATURES. 


59 


this  at  the  front  is  joined  to  segment  j  and  also  to  inductor 
No.  28.  Proceeding  in  this  way,  the  winding  scheme  may 
be  tabulated  as  follows  : 


REAR  END                   FRONT  END 

to         to         to 

10 

19 

j 

28 

28 

3 

b 

12 

12 

21 

k 

30 

3° 

5 

c 

14 

M 

23 

1 

32 

32 

7 

d        16 

16 

25 

m       34 

34 

9 

e 

18 

18 

27 

n 

2 

2 

ii 

f 

20 

20 

29 

o 

4 

4 

13 

g 

22 

22 

31 

P 

6 

6 

15 

h 

24 

24 

33 

q 

8 

8 

17 

i 

26 

26 

a 

10, 

Thus  the  winding  forms  one  closed  circuit,  and  it  is  evident 
that  there  are  two  paths  from  the  positive  to  the  negative 
brush.  It  is  a  progressive  winding. 

The  number  of  inductors  spanned  by  the  end  connections 
at  each  end  is  called  the  winding  pitch,  and  is  represented 
by  \.  In  Fig.  26  the  inductors  are  numbered  consecutively 
from  i  to  34  in  passing  around  the  armature,  and  inductor 
No.  10  joins  inductor  No.  19  at  the  rear;  the  rear-end 
winding  pitch  is  therefore  9,  or 

;r  =  19-  10  =  9. 

At  the  front,  or  commutator,  end,  inductor  No.  19  joins  in- 
ductor No.  28,  so  that  the  front-end  winding  pitch  is  also 

J/-9- 

It  occurs  frequently,  however,  that  the  front  and  rear  wind- 


6o 


DYNAMO   ELECTRIC   MACHINERY. 


ing  pitches  are  different,  in  which  case  the  mean  winding 
pitch  is  Xr  +  Xj 

A   ==  * 

2 

There  exists  a  definite  relation  between  the  total  number 
of  inductors  on  the  surface  of  an  operative  armature,  the 
number  of  pairs  of  poles,  and  the  winding  pitch.  For  the 
wave  winding,  the  permissible  total  number  of  inductors  is 

C=   2p\   ±   2. 

Hence,  for  the  four-pole  machine  of  Fig.  26  the  total 
number  of  inductors  would  be2X2X9±2;  that  is,  C  = 
38  or  34.  The  latter  number  was  here  chosen. 


Fig.  27. 


ARMATURES. 


61 


A  six-pole  wave  winding  for  a  drum  armature  having  31 
slots  is  shown  in  Fig.  27.  There  are  62  inductors,  the 
even  and  odd  numbered  ones  representing  the  lower  and 
upper  inductors  in  the  slots  respectively.  The  winding 
pitches  for  this  armature  are 

Xr  =  9, 
Xf=  n, 
and  X  =  10. 

A  six-pole,  retrogressive,  lap  or  multiple-circuit,  in  this 
case  six-circuit,  winding  is  shown  diagrammatically  in  Fig. 
28,  and  it  is  seen  that 
there  are  six  paths  in 
parallel  from  the  positive 
to  the  negative  brushes. 
There  are  80  inductors, 
the  winding  pitches 
being 

Xr  =  ii 

and  Xf  =  13. 
In  the  lap  winding,  the 
front  and  rear  winding 
pitches  cannot  be  equal 
and  the  difference  be- 
tween them  must  be  some  multiple  of  2.  If  there  be  two 
inductors  per  slot,  and  the  lower  ones  be  even-numbered 
and  the  upper  ones  odd-numbered,  as  usual,  then  the  front 
and  rear  winding  pitches  must  be  odd,  and  therefore  the 
mean  winding  pitch,  X,  is  always  an  even  number.  The 
expression  for  the  total  number  of  inductors  on  a  lap 
winding  for  /  pairs  of  poles  allows  more  latitude  than  is 
the  case  with  wave  windings,  and  is 

C  =  2     X. 


62  DYNAMO   ELECTRIC   MACHINERY. 

Herefrom  the  mean  winding  pitch  is 


but  frequently  values  of  X  are  taken  which  are  considerably 
smaller  than  the  value  obtained  from  this  formula.     In  the 

present  case,  for  example,  X  =  1  2  instead  of  —  .    Such  vvind- 

6 

ings  are  called  short-chord  windings. 

As  previously  stated,  a  winding  element  of  a  drum  arma- 
ture has  two  inductors,  yet  there  may  be  any  number  of 
turns  of  wire  in  the  element.  This  is  often  so  in  practice, 
where  high  terminal  voltages  are  desired.  Fig.  29  shows 


Fig.  29.  Fig.  30. 

a  three-turn  element  for  a  wave  winding,  and  Fig.  30  a 
similar  element  for  a  lap  winding.  Winding  pitches  are 
sometimes,  but  not  in  this  text,  expressed  by  the  number 
of  spanned  core-slots  or  subtended  angles  at  the  axis,  the 
latter  being  expressed  in  radians  or  degrees  either  electrical 
or  mechanical. 

37.    Multiplex    Armature    Windings.  —  In    the    simplex 
armature  windings  thus  far  considered,  a  winding  would 


ARMATURES.  63 

consist  of  one  complete  circuit.  In  multiplex  windings,  on 
the  other  hand,  there  may  be  two  or  more  distinct  circuits 
completely  insulated  from  each  other,  and  each  of  these 
might  be  provided  with  a  separate  commutator  and  set  of 
brushes.  The  usual  practice,  however,  is  to  provide  only 
one  commutator  with  the  segments  pertaining  to  one  wind- 
ing intermeshed  with  those  belonging  to  the  other  wind- 
ings. Thus  no  greater  number  of  brushes  is  required  than 
for  a  corresponding  simplex  winding,  yet  they  must  be 
considerably  wider  so  that  simultaneous  commutation  of 
all  the  circuits  may  be  going  on. 

Starting  from  a  certain  commutator  segment,  and  trav- 
ersing/ elements  of  a  simplex  wave  winding,  or  any  one 
element  of  a  simplex  lap  winding,  one  reaches  the  next 
adjacent  commutator  segment.  Proceeding  in  like  manner 
with  a  duplex  winding,  one  reaches  the  second  following 
segment  from  the  starting  point ;  and  similarly,  with  a 
triplex  winding,  one  reaches  the  third  following  segment, 
etc.,  the  intermediate  segments  being  connected  to  the 
other  windings.  Such  multiplex  windings,  which  consist 
of  a  number  of  complete  and  independent  simplex  wind- 
ings, are  multiply  re-entrant,  that  is,  each  individual  circuit 
re-enters  upon  itself  to  form  a  closed  circuit.  Multiplex 
armature  windings  may  be  singly,  doubly,  triply,  or  in 
general,  multiply  re-entrant. 

A  singly  re-entrant  winding  is  one  in  which,  by  succes- 
sive angular  advances,  the  entire  winding  is  traversed  before 
returning  to  the  starting  point.  A  doubly  re-entrant  wind- 
ing is  one  in  which  only  half  of  the  winding  is  traversed 
before  reaching  the  initial  segment.  Similarly,  in  a  triply 
re-entrant  winding  only  one-third  thereof  is  traversed.  The 
number  of  separate  circuits  on  an  armature  determines  the 


DYNAMO   ELECTRIC   MACHINERY. 


Fig.  31. 


degree  of  re-entrancy.     The  number  of  times  it  is  necessary 
to  pass  around  the  armature  in  traversing  a  complete  circuit 

must  not  be  confounded 
with  the  degree  of  re- 
entrancy. 

Fig.  31  depicts  a  six- 
pole,  retrogressive,  two- 
circuit,  singly  re-entrant 
duplex  winding  compris- 
ing 58  inductors.  A  four- 
pole,  retrogressive,  two- 
circuit,  triply  re-entrant 
triplex  winding,  having  66 
inductors,  is  shown  in 
Fig.  32.  A  duplex  wind- 
ing may  be  either  singly  or  doubly  re-entrant,  a  triplex 
winding  may  be  either  singly  or  triply  re-entrant,  and  a 
quadruplex  winding  may 
be  singly,  doubly,  or  quad- 
ruply  re-entrant.  Multi- 
plex windings  beyond 
these  are  rarely  used  in 
practice. 

The  general  formula  for 
multiplex  wave  windings 
is 

C  =  2  pi  ±  2  y, 

where   C,  p,  and  X  have 

the  same  significance  as 

before,  and  where  y  is  the 

multiplicity  of  the  winding,  whether  duplex,  triplex,  and  so 

on.      For  a  given   multiplex  winding,   the  choice  of  the 


ARMATURES.  6$ 

mean  winding  pitch,  and  therefore  also  the  total  number 
of  inductors,  depends  upon  the  degree  of  re-entrancy 
desired.  The  highest  common  factor  of  A  and  y  expresses 
the  degree  of  re-entrancy.  Thus,  for  the  duplex  winding 
of  Fig.  31,  y  =  2 

and  A  =  9  ; 

therefore  the  winding  must  be  singly  re-entrant,  since  the 
highest  common  factor  of  2  and  9  is  I.  The  total  number 
of  inductors  for  this  winding  might  be 

C=2X3X9±(2X2) 
=  50  or  58. 

The  latter  value  was  here  chosen. 
For  the  triplex  winding  of  Fig.  32, 

y  =  3          and          ^  =  1  5  ; 

hence  the  winding  is  triply  re-entrant.  The  total  number 
of  inductors  might  also  have  been  54  instead  of  66  as 
shown, 

In  multiplex  lap  windings,  the  degree  of  re-entrancy  is 
equal  to  the  highest  common  factor  of  half  the  number  of 

inductors,-,  and  the  multiplicity  of  the  winding,  y.  The 
mean  winding  pitch  is  chosen  as  near  as  possible  to 


the  same  as  for  simplex  lap  windings. 

38.  Equalizing  Connections.  —  The  electromotive  forces 
generated  in  different  sections  of  an  armature  are  not  ex- 
actly equal,  due  to  inaccurate  centering  of  the  armature 
and  to  the  unavoidable  differences  of  distance  of  the  arma- 
ture conductors  from  the  field  pole  pieces.  The  E.M.F. 


66  DYNAMO   ELECTRIC   MACHINERY. 

differences,  although  small,  nevertheless  set  up  local  cur- 
rents in  the  armature,  and  these  may  result  in  excessive 
heating  of  the  conductors  and  sparking  at  the  commutator. 
In  wave  windings,  little  difficulty  is  experienced  in  this 
respect,  because  the  inductors  are  connected  in  series  and 
are  distributed  under  all  the  poles;  but  in  lap  windings 
large  internal  currents  are  produced  in  the  armature  which 
cause  operative  troubles.  To  minimize  troubles  from  these 
currents,  lap-wound  armatures  of  large  generators  are  sup- 
plied with  equalizing  connections  of  low  resistance.  These 
are  connections  between  points  of  the  winding  which  should 
be  at  the  same  potential,  and  they  usually  take  the  form 
of  rings  situated  at  the  commutator  end  of  the  armature 
core. 

39.  E.M.F.  Equation  of  Dynamos.  —  The  average  elec- 
tromotive force  between  brushes,  induced  in  the  conductors 
of  an  armature  -revolving  in  a  multipolar  magnetic  field,  is 

£av  =  2  &mSf  io-8  volts,  §33 

where  4>wl  is  the  flux  per  pole  in  maxwells  which  cuts  the 
conductors,  »S  is  the  number  of  conductors  in  series  between 
brushes,  and /is  the  number  of  magnetic  cycles  through 

which  the  armature  core  passes  in  one  second  ;  or/  =  / . 

60 

This  equation  is  perfectly  general,  and  applicable  to  any 
type  of  direct-current  dynamo  with  any  style  of  armature 
winding.  To  ensure  its  proper  application,  a  consideration 
of  the  significance  of  the  term  5  for  various  styles  of 
winding  is  necessary. 

In  general,  vS  is  the  total  number  of  conductors  on  the 
armature  divided  by  the  number  of  current  paths  through 
the  armature  between  brushes.  Reference  to  §§36  and 


ARMATURES. 


67 


37  shows  that  the  number  of  paths  between  brushes  for 
various  styles  of  winding  are  as  follows  : 


TYPE   OF   WINDING 

WAVE 

LAP 

Simplex  . 

2 

2/ 

Duplex    . 

4 

4/ 

Triplex    . 

6 

6/ 

Quadruplex 

8 

*$ 

As  a  numerical  example,  compute  the  no-load  voltage  of 
an  8-pole  generator  having  a  simplex  lap-wound  armature 
with  a  total  of  1920  conductors,  when  revolving  at  300  rev. 
per  min.  The  effective  magnetic  flux  per  pole  is  7  mega- 
maxwells. 

There  are  4  pairs  of  poles,  whence  the -number  of  con- 
ductors per  circuit  is 


1920 


-  240. 


2X4 

The  number  of  magnetic  cycles  passed  through  per  second  is 

300 


/=4 


60 


=  20. 


Therefore  the  terminal  E.M.F.  of  the  generator  (neglect- 
ing the  resistance  drop  in  windings)  is 
7000000 


=  2 


I08 


240  •  20  =  672  volts. 


40.  Core  Construction. — To  reduce  to  a  minimum  the 
otherwise  excessive  eddy  current  loss  (§  29)  in  armature 
cores,  these  are  constructed  of  thin  discs  of  soft  wrought 
iron  or  mild  steel,  which  are  more  or  less  thoroughly  insu- 
lated from  each  other  by  the  natural  oxide  or  by  a  coating 
of  varnish  on  the  discs.  Sometimes,  for  special  machines, 
shellac  coatings  on  the  discs,  or  thin  paper  sheets  between 


68  DYNAMO   ELECTRIC   MACHINERY. 

them,  are  applied.  Laminating  the  core  in  this  way  does 
not  completely  prevent  the  flow  of  eddy  currents,  for  small 
E.M.F.'s  will  still  be  induced  in  each  lamina  which  produces 
them.  The  eddy  current  loss,  being  proportional  to  the 
square  of  the  thickness  of  laminations,  would  be  lowered  by 
using  very  thin  discs.  A  limit  to  the  reduction  of  thickness 
is  the  increased  loss  due  to  the  higher  flux  density  neces- 
sary, owing  to  waste  of  space  which  is  taken  up  by  the  in- 
sulation between  laminations. 


Fig.  33- 

For  the  smaller  machines,  having  armatures  less  than  16 
inches  in  diameter,  the  discs  are  punched  in  one  piece,  and 
usually  take  the  form  shown  in  Fig.  33.  Sometimes  aper- 
tures are  provided  in  the  laminations  about  the  axis,  and 


ARMATURES. 


69 


these  constitute  air  passages  through  the  core,  thus  im- 
proving ventilation.  These  discs  are  mounted  directly 
on  the  shaft  and  are 
keyed  to  prevent  turn- 
ing. They  are  held 
in  position  by  flanges  jfT  j  |  | 
of  cast  steel  or  cast 
iron,  which  are  pressed 
together  by  nuts  on 
the  shaft,  as  shown  in 

Fig.  34,  or  by  bolts  passing  through,  but  insulated  from, 
the  laminations. 

For  the  larger  armatures,  the  discs  consist  of  a  number 
of  segments  which  are  assembled  on  a  mechanical  support 


Fig.  35- 


called  a  spider,  being  attached  thereto  by  inwardly-project- 
ing lugs  on  the  segments.  In  Fig.  35  is  shown  in  part  the 
spider  of  a  3OO-K.W.  generator  with  a  segment  dovetailed 


70  DYNAMO   ELECTRIC   MACHINERY. 

to  one  of  the  arms.  The  joints  are  staggered  in  the  lami- 
nations of  successive  layers.  Fig.  36  illustrates  a  section 
of  the  spider  of  the  same  machine  with  the  end  flanges  hold- 
ing the  laminations  in  place.  The  flanges  are  shaped  so  as 
to  form  a  support  for  the  armature  winding.  The  spider, 
which  has  an  extension  for  supporting  the  commutator,  is 
pressed  onto  the  shaft  and  keyed. 


Fig.  36. 


To  obtain  sufficient  ventilation  in  the  armature  and 
thereby  lessen  the  temperature  rise  incident  to  operation, 
it  is  usual  to  provide  radial  ventilating  ducts  in  the  core. 
The  rotation  of  the  armature  causes  air  to  pass  in  through 
the  axial  apertures,  or  spider  openings,  and  out  through 
the  ventilating  ducts  as  indicated  by  the  arrows  in  Fig.  37. 
The  ducts  are  formed  by  separating  the  laminations  at 
intervals  by  the  interposition  of  blocks  of  non-magnetic 


ARMATURES. 


material  called  spacing  pieces.  A  type  of  spacing  piece  is 
shown  in  Fig.  38,  which  consists  of  brass  strips  set  edge- 
wise into  slots  punched  in  stout  core  discs  and  riveted. 


Fig.  37- 


This  type  is  commendable  because  it  gives  support  to  the 
armature  teeth. 

Good  practice  requires  the  provision  of  one  ventilating 
duct  for  every  2  to  4  inches  of  axial  core  length,  the  width 


Fig.  38. 


of  the  ducts  varying  from  three-eighths  to  five -eighths  of 
an  inch. 

Fig.  39  illustrates  a  Westinghouse  armature  core  partly 
assembled  on  the  spider.     One  of  the  segments  of  a  disc 


72  DYNAMO   ELECTRIC   MACHINERY. 

and  a  spacing  piece  are  shown  leaning  against  that 
portion  of  the  spider  upon  which  the  commutator  is  to 
be  mounted. 


Fig.  39- 

Two  types  of  armature  slots  are  in  general  use,  the 
open  slot  and  the  partly  closed  slot.  The  former  has  the 
advantage  that  the  armature  conductors  may  be  formed 
into  coils,  insulated,  and  readily  inserted  into  the  slots, 


whereas  the  latter  type  simplifies  the  matter  of  securing 
the  windings  into  place.  The  open  slot  is  more  often 
employed  on  direct-current  machines.  Fig.  40  shows  some 
of  the  styles  of  open  and  partially  closed  slots ;  the  re- 


ARMATURES.  73 

cesses  at  the  top  of  some  teeth  being  provided  for  the 
insertion  of  fiber  or  wooden  wedges,  which  serve  to  retain 
the  conductors  in  the  slots. 

In  many  machines  the  windings  are  held  in  place  by 
binding  wires  wound  around  the  periphery  of  the  armature. 
Grooves  are  provided  therefor  by  having  some  of  the  discs 
of  slightly  smaller  diameter.  The  wire  used  for  this  pur- 
pose is  generally  of  hard-drawn  brass  or  phosphor  bronze, 
and,  on  railway  motors,  of  steel.  It  is  wound  over  insulating 
strips,  forming  a  band  of  several  turns,  these  being  often 
soldered  together. 

41.  Armature  Coils.  —  The  armature  coils  are  of  copper, 
in  the  form  of  either  wire  or  strips,  the  former  being 
usually  employed  on  the  smaller  machines.  For  machines 
of  large  output,  it  is  not  advisable  to  use  heavy  bars  as 
conductors,  because  of  the  eddy  currents  set  up  in  them 
when  one  side  of  a  coil  is  momentarily  in  a  stronger  field 
than  the  other.  Thus,  a  number  of  smaller  conductors, 
insulated  from  each  other  and  connected  in  parallel  at 
the  commutator,  avoid  this  condition. 

In  multipolar  armatures  the  windings  consist  of  a  num- 
ber of  similar  and  interchangable  formed  coils,  which  are 
wound  on  separate  collapsible  forming  blocks.  The  several 
conductors  that  constitute  one  coil  are  insulated  individually 
and  are  fastened  together  and  wrapped  with  a  few  layers 
of  insulating  tape.  For  this  purpose  cotton  or  linen  tape, 
varnished  cloth  or  paper,  or  micanite  are  generally  used. 
The  advantage  of  formed  coils  is  their  superior  insulation 
and  the  facility  with  which  damaged  or  burned-out  coils 
can  be  removed  without  disturbing  the  other  coils.  Fig. 
41  shows  some  Western  Electric  Company  formed  coils, 
those  on  the  right  being  for  lap-wound  and  those  on  the 


74 


DYNAMO   ELECTRIC   MACHINERY. 


left  for  wave-wound  armatures.  In  Fig.  42  is  depicted  an 
armature  core  ready  for  winding,  with  some  formed  coils 
in  different  stages  of  completion. 

One-turn  armature  coils  for  large  dynamos,  having  con- 
ductors of  large  sectional  area,  which  do  not  permit  of 
bending,  are  composed  of  two  halves  individually  insulated 
and  placed  in  their  respective  slots.  A  copper  bridge  is 
then  bent  over  the  bare  ends  at  the  rear,  and  soldered, 
thus  completing  the  electrical  continuity  of  the  coil. 


Fig.  41. 

Before  placing  the  armature  coils  in  the  slots,  the  latter 
are  generally  lined  with  insulating  material,  such  as  mica- 
nit  e,  fiber,  and  a  paper  pulp  known  as  presspahn.  The 
thickness  of  slot  insulation  depends  upon  the  terminal  volt- 
age of  the  machine,  and  should  be  capable  of  withstanding 
several  times  this  voltage  without  puncturing.  On  the 
other  hand,  the  thickness  should  not  be  so  great  as  to 
materially  lower  the  space  factor  of  the  slot,  i.e.  the  ratio 
of  the  copper  cross-section  to  the  slot  area.  This  factor 
depends  upon  a  number  of  conditions,  but  common  values 


ARMATURES. 


75 


for  machines  up  to  75  K.W.  at  voltages  from  100  to  600 
may  be  interpolated  from  the  curves  of  Fig.  43  given  by 
Hobart. 


A  partly-wound  armature  for  a  2OO-K.W.,  5oo-volt  gen- 
erator manufactured  by  the  Allis-Chalmers  Company  is 
shown  in  Fig.  44.  After  all  the  coils  are  in  place,  the 


DYNAMO   ELECTRIC   MACHINERY. 


exposed  portions  of  the  windings,  or  the  end-connec- 
tions, must  be  firmly  bound  to  withstand  the  centrifugal 
force. 

42.  Commutators.  —  The  segments  or  bars  of  a  commu- 
tator are  always  of  drop-forged  or  hard-drawn  copper. 
They  must  be  properly  tapered  so  that  when  all  the  seg- 
ments are  put  together  the  whole  will  form  a  cylindrical 
structure.  The  insulation  between  segments  is  always  of 
mica.  Of  the  various  grades  of  mica  employed  for  insulat- 
ing purposes,  the  amber-colored  mica,  which  must  be  free 


60 


80 


0  20  40 

KILOWATTS 

Fig.  43- 

from  iron,  is  to  be  preferred.  Besides  being  a  good  insu- 
lator, amber  mica  has  the  additional  advantage  of  wearing 
at  the  same  rate  as  copper;  thus  after  long  use  it  leaves 
neither  elevations  nor  depressions  on  the  commutator  sur- 
face. Not  only  must  the  individual  segments  be  well 
insulated  from  each  other,  but  especially  good  insulation 
must  be  provided  between  the  segments  and  the  spider 
upon  which  they  are  mounted  and  the  clamping  rings 
which  hold  them  in  position,  because  the  potential  differ- 
ences at  these  places  are  the  same  as  the  terminal  voltage 


ARMATURES. 


77 


of  the  machine.     The  usual  thicknesses  of  mica  required 
for  commutator  insulation,  in  inches,  are  : 


VOLTAGE    OF    MACHINE 

300  VOLTS   OR    LESS 

300-1000  VOLTS 

Between  adjacent  Segments  .     .     . 
Between  Segments  and  Spider  .     . 

O.O2  tO  O.O4 
O.o6  tO  O.I  2 

0.04  to  0.06 
o.io  to  0.15 

Fig.  44. 

When  the  commutator  segments  and  mica  strips  are 
assembled  in  place,  a  pair  of  temporary  steel  rings  is  placed 
around  them,  the  inner  one  being  split  into  a  number  of 
sections,  as  shown  in  Fig.  45.  The  screws  are  tightened 
so  that  the  component  parts  of  the  commutator  are  firmly 
pressed  together.  A  groove  is  then  turned  therein,  and 
clamping  rings  of  corresponding  shape  are  fitted.  These 


?8  DYNAMO  ELECTRIC   MACHINERY. 

are  then  bolted  on,  and  the  steel  rings  removed,  leaving 
a  completed  commutator,  such  as  shown  in  section  in 
Fig.  46.  Considerable  reliance  is  placed  on  the  clamping 
rings,  for  these  must  prevent  the  possible  dislocation  of  the 
segments  due  to  expansion  and  contraction  which  accom- 
pany temperature  change,  or  due  to  centrifugal  force. 


n 


Fig.  45- 


Fig.  46. 


To  secure  successful  operation  a  commutator  must  be 
designed  with  a  sufficient  number  of  bars,  so  that  the 
difference  of  potential  between  two  adjacent  bars  shall 
not  exceed  10  volts.  This  would  mean  that  a  loo-volt 
bipolar  machine  should  have  at  least  20  bars.  The  poten- 
tial between  the  brushes  or  around  half  the  commutator 
is  100  volts,  hence  half  the  commutator  must  have  ten 
bars. 


ARMATURES.  79 

The  number  of  commutator  segments  to  be  used  depends 
upon  the  style  of  winding  and  the  voltage  of  the  machine. 
According  to  Arnold,  this  number  should  never  be  less 
than  0.037  times  the  product  of  the  total  number  of  arma- 
ture conductors  and  the  square  root  of  the  current  per 
armature  circuit.  The  width  of  a  commutator  segment 
for  good  mechanical  construction  should  not  be  less  than 
T\  inch  at  the  periphery. 

Commutators  for  turbo-generators  or  other  high-speed 
dynamos  usually  have  small  diameters,  so  that  the  linear 
speed  is  not  excessive ;  nevertheless  speeds  as  high  as 
8000  ft.  per  min.  are  encountered  in  practice,  which  is  two 
or  three  times  as  great  as  that  of  the  usual  low-speed  com- 
mutators. The  ordinary  methods  of  commutator  construc- 
tion for  these  high  speeds  are  inadequate  because  of  the 
great  centrifugal  force  tending  to  pull  the  commutator 
apart.  To  prevent  this  action,  stout  steel  rings,  well  in- 
sulated from  the  segments,  are  shrunk  on  the  outside  of 
the  commutator  at  several  places.  High-speed  commu- 
tators have  a  great  axial  length  in  order  to  secure  a  large 
radiating  surface. 

Commutators  should  be  designed  with  sufficient  exposed 
area  so  as  to  radiate  the  heat  which  is  communicated  to 
them  without  too  high  a  temperature  elevation.  The  total 
commutator  loss  consists  of  two  principal  components,  the 
loss  due  to  resistance,  and  that  due  to  friction. 

The  transition  resistance  between  the  brushes  and  com- 
mutator causes  a  drop  in  voltage  at  each  point  of  contact. 
This  drop  depends  upon  the  quality  of  the  brush,  and  is 
practically  independent  of  the  linear  speed  of  the  commu- 
tator, the  current  density  at  brush  contacts,  and  brush 
pressure.  The  drop  for  different  grades  of  brushes  varies 


80  DYNAMO   ELECTRIC   MACHINERY. 

between  0.6  and  1.4  volts  for  the  contacts  of  one  polarity. 
Double  these  values  times  the  current  output  of  the  ma- 
chine gives  the  loss  represented  by  the  transition  resistances. 
The  pressure  of  the  brushes  on  the  commutator  should 
be  low,  thus  resulting  in  a  small  friction  loss.  Light  pres- 
sure does  not  materially  increase  the  voltage  drop  at  the 
brushes.  The  magnitude  of  the  friction  loss  in  watts  is 

equal  to  ( -  — J  times  the  product  of  the  following 

quantities :  the  radius  of  the  commutator  in  feet,  the  speed 
in  revolutions  per  minute,  the  coefficient  of  friction  between 
the  brushes  and  the  commutator  (0.3  for  carbon  brushes 
and  0.25  for  copper  brushes),  and  the  sum  of  the  pressures 
of  all  the  brushes  upon  the  commutator.  This  latter  should 
amount  to  1.25  Ibs.  per  square  inch  of  rubbing  surface. 
Carbon  brushes  permit  a  current  density  of  30  to  70  am- 
peres per  square  inch  of  rubbing  surface,  and  copper  brushes 
about  eight  times  as  much. 

There  is  an  additional  loss  at  the  commutator  due  to  the 
sparking  at  the  brushes  and  to  the  currents  in  the  short- 
circuited  segments.  These  losses  cannot  be  calculated 
closely,  but  may  be  estimated  as  equal  to  about  six  per 
cent  of  the  regular  commutator  losses. 

Knowing  the  total  losses  in  the  commutator  the  tem- 
perature rise  may  be  estimated  by  means  of  empirical  ex- 
pressions, §  68. 

The  connections  from  the  armature  windings  to  the 
commutator  segments  are  made  by  means  of  metal  strips 
called  risers,  which  are  firmly  clamped  and  soldered  to  the 
rear  ends  of  the  segments.  In  Fig.  47  is  shown  a  completed 
Western  Electric  Company  commutator  with  the  risers 
attached. 


ARMATURES.  8l 

After  a  commutator  has  been  in  use  for  a  time,  it  becomes 
grooved  and  pitted,  a  condition  which  causes  further 
sparking  and  wear,  and  the  commutator  must  be  turned 
down  again  to  a  true  surface.  The  design  of  a  com- 
mutator should  allow  sufficient  material  for  repeated  opera- 
tions of  this  kind. 


Fig.  47- 

43.  Brushes  and  Brush  Holders.  —  Brushes  are  gener- 
ally made  of  hard  blocks  of  graphitic  carbon.  These 
brushes -wear  well  mechanically  and  give  the  commutator 
a  smooth  surface,  and  further,  the  greater  resistance  of  a 
carbon  brush  results  in  less  sparking  when  it  bridges  two 
commutator  bars  than  would  the  lower  resistance  of  a 
copper  brush.  Carbon  brushes  are  generally  set  at  an 
angle,  though  some  makers  set  them  radially,  especially  in 
motors  which  must  be  reversed  in  direction,  as  in  the  case 
of  railway  and  elevator  motors. 


82 


DYNAMO   ELECTRIC   MACHINERY. 


On  low-potential  machines  brushes  of  copper  gauze  are 
sometimes  used,  because  there  is  less  tendency  to  spark 
on  low  voltages,  and  because  the  resistance  of  carbon 
brushes  would  be  too  great  a  portion  of  the  resistance  of 
the  entire  circuit.  Such  brushes  are  also  used  on  some 
turbo-generators. 

The  number  of  brush  sets  necessary  depends  upon  the 
style  of  winding  employed  on  the  armature,  §  36 ;  but 
there  may  be,  and  usually  are,  several  brushes  per  set. 
That  is,  instead  of  broad  brushes,  a  number  of  smaller 
ones  are  used  on  all  machines  except  those  of  little  out- 
put. This  scheme  enables  the  removal  of  brushes  one 
at  a  time  for  trimming  purposes  while  the  machine  is  in 
operation. 

Individual  brushes  are  supported  in  brush  holders,  as  in 
Fig.  48,  which  shows  a  box-guide  type  of  Westinghouse 

manufacture.  Brush 
holders  should  provide 
adjustment  as  to  posi- 
tion and  tension  of  the 
brushes,  and  allow  the 
latter  to  follow  any  ir- 
regularity in  the  com- 
mutator surface.  The 
brush-holder  springs 
should  be  arranged  so 
that  the  brush  pressure 
is  maintained  constant 
during  the  life  of  the  brush.  The  spring  should  not 
form  a  part  of  the  electric  circuit ;  flexible  copper  con- 
ductors generally  complete  the  connection  from  the  brush 
to  the  stationary  part  of  the  holder.  A  General  Elec- 


Fig.  48. 


ARMATURES.  83 

trie    Company   brush-holder    arm    is    shown    complete    in 
Fig.  49. 

The  brush-holder  arms  are  carried  on  rings,  called 
rockers,  which  are  mounted  concentric  with  the  commuta- 
tor, either  on  a  sleeve  at  the  front  bearing  or  on  the  field 


Fig.  49- 

magnet  frame.  The  rocker  is  capable  of  being  moved 
around  the  commutator  by  means  of  a  tangential  screw 
and  hand  wheel.  After  the  proper  brush  position  has  been 
found  the  rocker  is  securely  clamped.  All  the  brushes,  both 
positive  and  negative,  are  usually  mounted  on  the  same 
rocker,  so  that  their  adjustment  is  simultaneously  effected. 
Fig.  50  shows  such  a  rocker  with  the  brush  gear. 

44.  Shafts  and  Bearings. —  As  shafts  for  armatures  are 
often  subjected  to  sudden  large  variations  in  load,  it  is 
usual  to  construct  them  somewhat  larger  than  those  of 
other  machines  of  similar  size.  The  diameter  of  the  shaft 
depends  upon  the  output  and  speed  of  the  armature,  and 
to  obtain  practical  values  the  following  empirical  formula 
may  be  used : 


4/K.W. 
V  rev.  p 


output 


per  mm. 


84 


DYNAMO   ELECTRIC   MACHINERY. 


Fig.  50. 


Fig.  51. 


ARMATURES.  85 

where  Ds  is  the  shaft  diameter  in  inches,  and  k  is  a  con- 
stant having  the  following  values  for  different-sized  machines 
using  mild-steel  shafts  : 

50  K.W.  or  less,  k=    6.5 

50  K.W.  to  sooK.W.,       k=     8.4 

500  K.W.  and  over,  k  =  10.2 

This  shaft  diameter  refers  to  that  part  under  the  core  and 
commutator,  the  portions  within  the  bearings  being  some- 
what less. 

Dynamo  bearings  should  have  ample  bearing  surface  and 
be  rigidly  constructed.  They  are  always  made  in  two  sec- 
tions, thus  permitting  the  removal  of  the  armature.  It  is 
necessary  that  the  bearings  be  exactly  in  line,  and  frequently 
a  form  of  self-alignment  bearing  is  used,  as  in  Fig.  5  i.  The 
shaft  revolves  in  a  cylindrical  brass  bearing  having  an  outer 
spherical  enlargement  at  the  center  which  rests  upon  a  cor- 
responding bed  of  Babbitt  metal. 

Lubrication  may  be  secured  by  the  use  of  ordinary  oil 
cups,  but  generally  by  the  employment  of  self -oiling  devices. 
One  of  these,  Fig.  51,  consists  of  two  brass  rings  playing 
in  semi-circumferential  slots  in  the  bearing,  which  permit 
the  rings  to  hang  loosely  on  the  shaft.  The  bearing  pedes- 
tals are  hollow  under  the  rings  and  serve  as  oil  receptacles. 
As  the  shaft  revolves,  the  rings  also  revolve  at  such  a  rate 
as  to  carry  a  steady  stream  of  oil  up  into  the  slots,  thereby 
lubricating  the  bearing. 

Recently  ball  bearings  have  been  applied  to  bearings 
of  small  dynamos,  say  up  to  50  K.W.,  and  have  given 
complete  satisfaction.  One  type  of  ball-bearing  is  shown 
in  Fig.  52,  where  o  is  the  outer  ring  and  i  the  inner  ring 


86 


DYNAMO   ELECTRIC   MACHINERY. 


Fig.  52. 


with  the  hardened  polished  steel  balls, 
b,  between  them.  A  removable  piece,  /, 
somewhat  longer  than  the  diameter  of  a 
ball,  permits  of  the  insertion  of  the 
balls,  and  may  be  held  in  place  by  the 
screw  s.  The  rings  are  of  hardened  steel 
and  have  polished  running  surfaces.  The 
inner  ring  is  rigidly  fastened  to  the 
shaft. 

PROBLEMS. 


1.  The  instantaneous  E.M.F.  induced  in  a  conductor  of  a 
revolving  loop  at  the  moment  it  cuts  a  certain  magnetic  flux  at 
an  angle  of  60°  is  2.5  volts.     What  electromotive  force  is  in- 
duced in  this  conductor  at  the  instant  the  angle  between  the 
paths  of  flux  and  conductor  is  70  degrees  ? 

2.  The   maximum  E.M.F.  induced  in  each  of  six  similar 
loops    placed  30    electrical  degrees  apart,   revolving  together 
about  a  common  axis  in  a  uniform  magnetic  field  is  25  volts. 
Calculate  the  maximum  and  minimum  values   of  the  total  vol- 
tage resulting  from  the   connection  of  all  the  loops  in  series. 
Determine  the  percentage  fluctuation. 

3.  How  many  magnetic  cycles  does  the  armature  core  of  a 
i6-pole  dynamo  pass  through  per  second,  if  the  armature  makes 
375  revolutions  per  minute? 

4.  How  many  inductors  may  there  be  on  a  i2-pole  simplex 
wave  winding  in  which  the  rear  winding  pitch  is   13  and  the 
front  winding  pitch  is  1 7  ? 

5.  There  are  122  inductors  on  an  eight-circuit  short-chord 
simplex   winding   which   is   imbedded   in    61    armature    slots. 
Determine  the  maximum  value  of  the  mean  winding  pitch,  the 
even-numbered  conductors  being  located  in  the  bottoms  of  the 
slots. 


PROBLEMS.  87 

6.  A  six-pole  multiplex  wave  winding  has  60  inductors  and 
a  mean  winding  pitch  of  9.     What  is  the  degree  of  multiplicity 
and  of  re-entrancy  of  the  winding  ? 

7.  The  armature  of  a  12 -pole  generator  makes  250  revolu- 
tions per  minute.     It  is  simplex  lap-wound,  and  has   180  slots 
with  4  conductors  per  slot.     What  is  the  average  value  of  the 
induced  E.M.F.  at  no  load  if  the  magnetic  flux  passing  through 
the  armature  is  10  megamax wells  per  pole? 

8.  A  triplex  wave-wound  armature  with  294  inductors,  each 
of  four  conductors,  is  substituted  for  the  armature  of  Prob.  7, 
all  other  conditions  remaining  the  same.     What  is  the  no-load 
terminal  voltage  ?     State  the  degrees  of  re-entrancy  obtainable 
in  this  armature  winding. 

9.  Determine  the  total  commutator  losses  of  a  250  rev.  per 
min.  dynamo  when  delivering  a  current  of  300  amperes.     Two 
positive  and  two  negative  sets  of  carbon  brushes  are  employed, 
the  grade  of  the  brushes  being  such  as  to  result  in  a  drop  of 
i .  i  volts  at  the  contacts  of  one  polarity.     A  current  density  of 
50  amperes  per  sq.  in.  of  brush  rubbing  surface  is  to  be  allowed. 
The  diameter  of  the  commutator  is  18  inches. 


88  DYNAMO   ELECTRIC   MACHINERY. 


CHAPTER  IV. 

FIELD    MAGNETS. 

45.  Field-Magnet  Frames.  —  In  the  foregoing  chapter 
was  shown  the  dependence  of  the  electromotive  force  in- 
duced in  a  dynamo  armature  upon  the  total  magnetic  flux 
cut  by  the  conductors.  This  magnetic  flux  is  produced  by 
the  current  in  the  field  coils  of  the  machine.  The  path  of 
the  flux  is  called  the  magnetic  circuit,  and  may  be  divided 
into  three  main  portions,  namely:  the  iron  of  the  field  mag- 
nets, the  armature  core,  and  the  air  gap  between  armature 
and  field-magnet  poles.  The  first  of  these,  which  consti- 
tutes the  field-magnet  frame,  may,  in  most  machines,  be 
subdivided  into  three  parts,  viz. :  ( i )  the  field  cores,  upon 
which  the  field  coils  are  situated;  (2)  the  yoke,  which  con- 
nects the  field  cores  at  the  outer  ends;  and  (3)  the  pole 
pieces  or  pole  shoes,  which  are  the  enlarged  inner  ends  of 
the  field  cores. 

The  frames  of  direct-current  generators  may  be  cast  in 
one  piece  either  of  cast  iron  or  cast  steel,  but  it  is  usual'  to 
construct  the  field  cores  separately  from  the  yoke.  The 
choice  of  material  for  the  yoke,  as  also  for  the  field  cores, 
is  governed  by  considerations  of  (a)  weight,  (b]  first  cost, 
and  (<:)  economy  and  satisfactory  regulation  in  operation. 
Cast  iron  has  the  advantage  over  cast  steel  in  cheapness, 
but  as  it  is  magnetically  inferior,  more  material  is  necessary 
to  carry  the  required  magnetic  flux;  further,  if  the  field 


FIELD    MAGNETS.  89 

poles  are  also  of  cast  iron,  the  expenditure  for  copper  will 
be  greater,  because  more  turns  will  be  required  and  each 
turn  would  be  longer  than  if  the  better  cast  steel  were  used. 
In  machines  having  different  parts  of  the  field  frame  of 
different  materials,  wrought  iron,  which  is  the  best  available 
magnetic  substance,  is  often  employed  in  the  form  of  punch- 
ings  for  the  cores  and  pole  pieces. 

For  multipolar  machines  the  yoke  is  generally  circular  in 
shape,  of  rectangular  or  elliptical  section,  and  is  divided 
either  horizontally  or  vertically  into  two  parts  to  facilitate 
the  removal  of  the  armature,  the  two  halves  normally  being 
bolted  together.  It  is  mounted  upon  a  cast-iron  bed  plate, 
to  which  are  also  fastened  the  pedestals  which  carry  the 
armature  bearings.  The  bipolar  type  of  machine  is  now 
restricted  to  the  high-speed  smaller  units,  say  5  K.W.  or 
less,  because  of  the  greater  amount  of  necessary  material 
occasioned  by  the  longer  magnetic  circuit.  Bipolar  field- 
magnet  frames  are  made  in  a  great  variety  of  shapes,  some 
of  which  are  shown  in  Figs.  21,  22,  and  23;  and  are  gen- 
erally cast  in  one  or  more  pieces  which  are  bolted  together 
after  the  field  coils  are  in  place. 

Separate  cores  for  multipolar  frames  are  constructed 
of  cast  steel,  laminated  wrought  iron,  or  laminated  steel. 
Solid  cores  are  usually  of  circular  or  of  rectangular  cross- 
section,  and  laminated  cores  are  of  the  latter  only.  A  pole 
piece,  built  up  of  soft-steel  laminations  riveted  together 
between  stout  end  plates,  is  shown  in  Fig.  53,  which  is 
representative  of  Westinghouse  practice.  Field  cores  may 
either  be  fastened  to  the  yoke  by  bolts  passing  through 
the  frame  and  screwed  into  the  core  or  pole  pieces,  or  be 
cast  integral  with  the  yoke.  The  latter  method  gives  es- 
pecially good  magnetic  joints,  but  the  former  allows  the 


90  DYNAMO   ELECTRIC   MACHINERY. 

removal  of  any  one  of  the  cores  with  its  field  coil  for  pur- 
poses of  repair. 

The  pole  tips  are  generally  somewhat  larger  than  the 
cores,  as  shown  in  Fig.   53,  an  arrangement  which  serves 


Fig.  53 

the  double  purpose  of  producing  a  more  uniform  flux  dis- 
tribution in  the  air  gap,  and  retaining  the  field  coils  in 
position.  In  this  particular  type,  one  corner  of  each 
punching  is  cut  away  and  the  laminations  are  stacked  with 
the  beveled  corners  alternately  to  one  side  and  to  the  other, 
thus  producing  a  pole  with  saturated  pole  tips,  which  is 
advantageous  in  yielding  good  commutation.  Where  solid 
poles  of  cast  iron  or  cast  steel  are  used  with  separate  pole 
pieces,  the  latter  are  preferably  made  of  laminated  wrought 
iron,  thereby  reducing  the  detrimental  effects  of  eddy  cur- 
rents induced  in  the  pole  faces  by  the  flux  variation  occa- 
sioned by  the  slots  in  the  revolving  armature  core. 

In  the  usual  designs  of  direct-current  dynamos,  the  ratio 
of  the  peripheral  length  of  the  pole  face  to  the  pole  pitch, 


FIELD    MAGNETS.  91 

or  distance  between  adjacent  pole  centers,  ranges  from  0.60 
to  0.75,  and  this  ratio  is  known  as  the  polar  span;  that  is, 
the  polar  span  is  from  60%  to  75%  of  the  pole  pitch. 


Fig.  54- 


Fig.  54  shows  the  complete  field-magnet  frame  of  a 
Crocker- Wheeler  generator.  This  frame  is  of  cast  iron, 
and  the  round  poles  are  of  cast  steel  provided  with  remov- 
able cast-iron  shoes,  which  are  clamped  in  place  after  the 
field  coils  have  been  assembled. 


DYNAMO   ELECTRIC   MACHINERY. 


46.  Methods  of  Field  Excitation.  —  Dynamos  are  classi- 
fied according  to  the  five  methods  of  exciting  the  fields  of 
the  machine.  They  are  :  —  the  Magneto,  the  Separately 
Excited,  the  Shunt  Wound,  the  Series  Wound,  and  the 
Compound  Wound.  The  last  three  methods  refer  to  self- 
exciting  machines,  that  is,  generators  which  supply  their 
own  field  current. 

The  magneto  generator,  Fig.  55,  is  one  in  which  the 
field  is  a  permanent  steel  magnet,  generally  of  horseshoe 
form.  Because  of  the  low  flux  densities  in  this  type  of 


MAGNETO  DVNAMO 
Fig'    55- 


SEPARATELY  EXCITED  DYNAMO 

Fig.  56. 


machine  necessitating  more  iron,  its  use  is  limited  to  small 
dynamos,  such  as  those  used  in  gasolene  engine  ignition 
work,  or  for  telephone  signaling. 

The  separately  excited  dynamo,  Fig.  56,  has,  as  its  name 
implies,  its  field  coils  traversed  by  a  current  other  than 
that  produced  by  the  machine.  It  is  produced  by  an  aux- 
iliary generator  called  an  exciter.  Alternating-current  ma- 
chines are  nearly  always  of  this  type. 

The  shunt -wound  machine,  Fig.  57,  has  a  large  number 
of  turns  of  fine  wire  wound  on  its  field  core,  and  the  ends 
are  connected  to  the  terminals  of  the  machine,  thus  being 


FIELD    MAGNETS. 


93 


in  shunt  with  the  outside  circuit.  The  ampere-turns  requi- 
site for  excitation  are  obtained  by  passing  a  small  current, 
say  from  2  to  8  per  cent  of  the  total  current  output,  through 
a  large  number  of  turns. 


SHUNT  WOUND 
DYNAMO 


Fig.  57- 


SERIES  WOUND 
DYNAMO 

Fig.  58. 


The  series-wound  generator,  Fig.  58,  has  all  the  current 
that  is  produced  by  the  armature  passing  through  a  few 
turns  of  large  wire  wound  around  the  field  cores.  The 
exciting  coils  are  then  in  series  with  the  external  circuit. 
The  ampere-turns  required  for  excitation  are  obtained  by 
passing  a  large  current  through  a  small  number  of  turns. 
Series  generators  are  practically  only  used  for  series  arc 
lighting  and  in  the  Thury  system  of  direct-current  power 
transmission  at  high  voltages. 

The  compound-wound  machine,  Fig.  59,  is  one  in  which 
there  are  both  shunt  and  series  coils  on  the  field  magnets. 
This  method  of  winding  is  used  for  purposes  of  regulation 
under  varying  loads,  as  will  be  explained  later.  Compound 
windings  are  of  two  classes,  the  long  shunt  and  the  short 


94 


DYNAMO   ELECTRIC   MACHINERY. 


shunt.  In  the  former,  the  current  used  in  the  shunt  wind- 
ings is  also  passed  through  the  series  windings  along  with 
the  main  current.  In  the  latter,  the  current  from  the 
shunt  coils  passes  directly  back  to  the  armature,  avoiding 


COMPOUND  WOUND 
DYNAMO  LONG  SHUNT 


COMPOUND  WOUND 
DYNAMO  SHORT  SHUNT 


Fig.  59- 


Fig.  60. 


the  series  turns.  Figs.  59  and  60  clearly  show  the  con- 
nections of  the  two  types.  The  short-shunt  compound 
winding  is  generally  preferred. 

Shunt-wound  and  compound-wound  generators  find  their 
principal  utilization  in  constant-potential  systems  of  elec- 
trical distribution  for  lighting  and  power. 

47.  Magnetic  Leakage.  —  Since  air  is  not  an  insulator 
of  magnetism,  but  is  simply  much  less  permeable  than 
iron,  it  is  evident  that  some  of  the  lines  of  force  generated 
by  the  field  coils  will  not  follow  around  the  desired  path 
through  pole  pieces  and  armature,  but  will  take  a  path 
through  the  air  and  be  of  no  utility  in  creating  E.M.F.  in 
the  revolving  armature.  Fig.  61  roughly  represents  some 
of  the  paths  such  lines  may  take. 


FIELD    MAGNETS. 


95 


If  &t  be  the  total  flux  set  up  by  the  field  coils,  and  <J>a  be 
that  portion  of  it  which  passes  through  the  armature  and 
is  cut  by  the  conductors,  then  the  coefficient  of  magnetic 
leakage,  or  dispersion  coefficient,  is 

,    *, 

*•*: 

and  is  always  greater  than  unity. 


Fig.  61. 


In  practice,  the  value  of  d  ranges  from  1.25  to  1.5  in 
bipolar  dynamos,  depending  upon  the  design.  For  multi- 
polar  machines,  the  values  of  the  dispersion  coefficient 
vary  from  1.3  in  small  dynamos  of  about  2  K.W.  to  i.i 
in  large  machines  of  about  500  K.W.  output. 

Increasing  the  field  excitation  of  a  generator  results  in 
more  magnetic  flux,  not  all  of  which  is  useful  in  developing 
a  greater  voltage.  Further,  the  leakage  flux  increases 
faster  than  the  useful  flux  ;  for  as  the  flux  density  of  the 
magnetic  circuit  becomes  greater  its  permeability  drops, 
whereas  the  permeability  of  air  is  constant  and  unity  at 
all  flux  densities.  The  dispersion  coefficient  is  therefore 
dependent  upon  the  load  on  the  machine. 


96  DYNAMO   ELECTRIC   MACHINERY. 

The  dispersion  coefficients  of  small  machines  for  definite 
excitation  may  be  determined  experimentally  by  the  use  of 
test  coils  in  connection  with  a  ballistic  galvanometer. 

48.  Calculation  of  Exciting  Ampere -Turns.  —  In  order 
that  a  generator  armature  may  produce  the  desired  voltage 
when  revolving  at  a  definite  speed,  a  certain  amount  of 
magnetic  flux  must  be  cut  by  the  armature  conductors. 
This  flux  is  set  up  in  the  magnetic  circuit  of  the  machine 
by  a  current  flowing  through  the  field  coils.  The  strength 
of  current  necessary  and  the  number  of  turns  of  wire  to  be 
provided  on  the  field  coils  depend  upon  the  length  and 
reluctance  of  the  magnetic  circuit  and  the  dispersion  coef- 
ficient. As  the  reluctances  of  the  various  portions  of 
the  magnetic  circuit  are  different  owing  to  differences  of 
dimensions,  flux  density,  or  permeability,  it  is  necessary  to 
calculate  the  magnetomotive  force,  or  ampere-turns,  for 
each  of  the  sections;  their  summation  will  then  determine 
the  required  exciting  ampere-turns,  after  correcting  for 
magnetic  leakage. 

In  designing  the  magnetic  circuit  of  a  dynamo  every 
portion  of  it  should  have  sufficient  cross-section  to  carry 
the  total  flux  at  a  reasonable  flux  density.  It  is  assumed 
in  such  calculations  that  all  the  magnetic  leakage  occurs 
at  the  pole  faces  ;  hence  the  total  flux  which  passes  through 
the  armature  core  and  teeth  from  the  air  gap  is  the  useful 
flux,  <f>a,  and  that  which  passes  through  the  center  of  the 
field  cores  is  the  total  flux  produced,  or  3>y  =  §$>a.  This 
assumption  simplifies  the  calculation  without  affecting  the 
degree  of  accuracy  to  any  great  extent. 

In  the  following  table  are  given  the  various  parts  of  the 
magnetic  circuits  of  ordinary  direct-current  multipolar 
machines,  with  the  usual  materials  of  which  they  are 


FIELD    MAGNETS. 


97 


constructed,    and    the    common    values    of    flux    density 
therein. 


PORTION    OF 
MAGNETIC    CIRCUIT 

MATERIAL 

FLUX    DENSITIES 
KILOMAXWELLS    PER    SQ.    IN. 

Armature  Core 

Soft  Laminated  Iron 

70  to  no 

Armature  Teeth 

Soft  Laminated  Iron 

100  to  140 

Air  Gap 

Air 

40  to    60 

C 

Cast  iron 

30  to    50 

Field  Cores 

Cast  steel 

7O  tO   TOO 

( 

Soft  Laminated  Iron 

70  to  no 

Field-magnet  Yokes    j 

Cast  iron 
Cast  steel 

27  to    45 
70  to    90 

Having  decided  upon  the  dimensions  and  material  of 
each  portion  of  the  magnetic  circuit,  the  ampere-turns 
required  to  drive  the  flux  through  them  is  calculated  from 
the  expression 


nl  =  -  ——  -  0.313  —  >  §§  23-27 

47:2.54  fi 

where  for  a  given  portion  (R  is  the  reluctance  in  oersteds, 
/  is  the  length  in  inches,  (B  is  the  flux  density  in  maxwells 
per  square  inch,  <£  is  the  total  flux  in  maxwells,  and  /*  is 
the  permeability  of  the  material.  In  multipolar  machines 
it  is  only  necessary  to  consider  one  complete  magnetic 
circuit,  that  is,  for  one  pair  of  poles.  The  resulting  ampere- 
turns,  are  those  necessary  for  two  field  coils. 

As  an  example  determine  the  ampere-turns  per  pole 
required  to  send  a  flux  of  20  megamaxwells  through  the 
armature  of  the  35O-K.W.,  5oo-volt,  8-pole  generator 
shown  in  part  in  Figs.  62  and  63,  in  which  the  dimensions 
are  expressed  in  inches.  The  mean  path  of  the  flux  is 
shown  by  the  closed  dotted  line. 

Armature  Core.  To  allow  for  the  insulation  between 
the  armature  laminations  it  is  usual,  in  practice,  to  con- 


DYNAMO    ELECTRIC    MACHINERY. 


f 

a 
i 

""A  ' 
I 

1 

i 
I 

7 
1 

FIELD    MAGNETS.  99 

sider  that  it  occupies  10%  of  the  gross  length  of  iron. 
The  presence  of  ventilating  ducts  further  decreases  the 
net  iron  length.  The  cross-sectional  area  of  the  armature 
core  is 

Ac  =  (14  -  1.2)  X  0.9  (15  -  4  X  0.5)  =  150  sq.  in. 

In  multipolar  machines  of  the  usual  construction  there 
are  two  paths  for  the  flux  per  pole  through  the  armature 
core  arid  field  yoke  ;  hence  only  one-half  of  the  flux  per 
pole  passes  through  these  parts.  The  flux  density  in  the 
armature  core  is 

20,000,000 

($>c  =  -  —  =  66,600  maxwells  per  sq.  in. 

150  X  2 

The  permeability  of  the  core  at  this  flux  density,  as 
determined  from  Fig.  10,  is  fic  =  1650. 

The  mean  length  of  path  traversed  by  the  flux  is 
approximately  33  inches. 

Therefore  the  ampere-turns  required  'to  overcome  the 
armature  core  reluctance  are 


(nl)c  =  '        =  420. 


Armature  Teeth.  The  accurate  determination  of  the 
ampere-turns  necessary  to  send  the  magnetic  flux  through 
the  armature  teeth  is  very  complicated,  but  the  following 
practical  method,  involving  a  few  assumptions,  leads  to 
good  results.  The  number  of  teeth  under  a  pole  piece  is 

OQ 

21  -f-  -  —  =18.2.     Owing  to  the  fringing  of  the  flux  at  the 
240 

pole  tips,  not  merely  the  teeth  immediately  under  the  pole 
face  carry  the  flux  from  that  pole,  but  one  or  more  addi- 
tional teeth  may  assist.  Making  an  allowance  of  10  per 


100  DYNAMO   ELECTRIC   MACHINERY. 

cent  for  this  flux  fringing,  a  value  frequently  taken,  then 
the  number  of  armature  teeth  receiving  flux  from  one  pole 
is  18.2  X  i.i  =  20. 

As  the  teeth  are  wider  at  the  periphery  than  at  the  bot- 
tom of  the  slots,  their  sectional  area  will  be  different  at 
various  distances  from  the  axis  of  rotation.  It  is  usual  to 
employ  the  sectional  area  at  one-third  the  tooth  length  from 
the  narrow  end.  The  width  of  the  tooth  at  this  place  is 

:  1.2)! 

— , —      O.c6       0.57  inch. 

240 

The  net  cross-sectional  area  of  the  flux-carrying  teeth  per 
pole  is 

At  =  20  X  0.57  X  0.9  (15    -  4  x  0.5)       134  sq.  in. 

Armature  teeth  are  worked  at  high  flux  densities,  and 
at  these  densities  the  permeability  of  iron  is  not  very  high. 
Consequently  the  permeance  of  the  copper  or  air  between 
the  teeth  cannot  be  neglected,  and  a  correction  must  be 
made  therefor.  The  terms  apparent  and  corrected  flux 
densities  are  to  be  distinguished  in  this  connection.  Thus, 
the  apparent  flux  density  in  the  teeth  is 

20,000,000  „ 

(Ska  =  =  149,000  maxwells  per  sq.  in. 

J34 

The  corresponding  corrected  flux  density  may  be  obtained 
from  the  curves  of  Fig.  64  given  by  Hobart.  The  differ- 
ent curves  refer  to  different  ratios  of  tooth-width  to  slot- 
width.  For  the  dynamo  under  consideration,  in  which  the 
tooth-width  and  slot-width  are  practically  equal,  the  cor- 
rected tooth  flux  density  is 

(Bfc  =  138,000  maxwells  per  sq.  in. 


FIELD   MAGNETS'.    ? 


150 


|  E14° 

is1 

oc 

3£ 

G±3130 
°$ 

ef 

0  s 

UJ   O 

1  *120 


IDTH 
W  '=  SLOT-WIDTH 


T='TOOTH-WI 

W.=  SLOT-WI 


1 


120 


130  140  150  160 

APPARENT    FLUX   DENSITY   IN 
KILOMAXWELLS  PEB  SQ.IN. 


170 


Fig.    64. 


KILOMAXWELLS  PER  SQ.IN. 
££  M  u*  4^  *j 

0  0  0  0  O 

\ 

^ 

^> 

^^ 

KJ.O 

21.7 

a 

20.2 
18.6 

c 

p 

\ 

* 

^ 

^ 

\ 

\ 

/ 

S 

SI 

EET  IF 

ON 

2 

\ 

/ 

\ 

^ 

/ 

'  \ 

^ 

^-, 

^^ 

/ 

"7" 

—-  -*. 

—  •  



L 

200       400       600       800       1000      1200   £ 
*       20       40       60       80       100       120 

Fig.  65. 


-ELECTRIC   MACHINERY. 

The  permeability  of  the  iron  at  this  flux  density,  as 
determined  from  Fig.  65,  is  /«  =  30. 

The  length  of  two  teeth  is  /$  =  2  x  1.2  =  2.4. 

Hence  the  ampere-turns  required  to  overcome  the  re- 
luctance of  the  armature  teeth  are 

•      .       ()|/)t.  0.313  X  MX  '38.000  =34;o      •      if} 

^^>  Gap.  The  usual  practice  in  computing  the  flux 
density  in  the  air  gap  is  to  divide  the  total  flux  per  pole 
entering  the  armature  by  the  area  of  the  pole  face,  thus : 

20,000,000 

GL  =  -  -  =  <;6,ooo  maxwells  per  sq.  in. 

17  X  21 

The  radial  length  of  the  air  gap  is  0.3  inch. 

Since  the  permeability  of  air  is  unity  regardless  of  flux 
density,  the  ampere-turns  required  to  overcome  the  air-gap 
reluctance  are 

(nl)a  =  0.313  X  0.3  X  2  X  56,000  =  10,500, 

which  is  the  principal  component  of  the  total  number  of 
ampere-turns  to  be  provided  on  the  machine. 

Field  Cores.  Taking  a  value  of  1.15  for  the  dispersion 
coefficient,  the  total  flux  traversing  a  field  core  will  be 
20,000,000  X  1.15  =  23,000,000  maxwells.  The  sectional 
area  of  the  poles  is 

Ap  =  17  X  14  =  238  sq.  in. 
Therefore  the  flux  density  in  the  poles  is 

23,000,000 

(Bp  =  -       =  96,600  maxwells  per  sq.  in. 

238 

The  permeability  of  cast  steel  at  this  flux  density,  as 
determined  from  Fig.  1 2,  is  ^p  =  480. 


FIELD   MAGNETS.  103 

The  length  of  two  field  cores,  including  pole  pieces,  is 
35.4  inches. 

Therefore  the  ampere-turns  required  to  overcome  the 
reluctance  of  the  field  poles  are 


Field  Yoke.  The  cross-sectional  area  of  the  yoke  is 
12x25  =  300  sq.  in.,  and  the  flux  traversing  it  is  \  X  23,000,- 
000=  11,500,000  maxwells.  Therefore  the  flux  density  is 

11,500,000 

(&y  =  —  —  —  -  -  =  38,300  maxwells  per  sq.  in. 
300 

The  permeability  of  the  cast-iron  yoke  at  this  flux  den- 
sity, as  determined  from  Fig.  n,  is  ^=253. 

The  mean  length  of  path  traversed  by  the  flux  is  approxi- 
mately 54  inches. 

Hence  the  ampere-turns  required  to  overcome  the  yoke 
reluctance  are 

0.313  X  54  X  38,300  =6a 
253 

Summary.  The  ampere-turns  per  pair  of  poles  neces- 
sary for  overcoming  the  reluctance  of  the  entire  magnetic 
circuit  and  the  various  parts  of  it  are  as  follows  :  — 

Armature  Core,  420 

Armature  Teeth,  3>45O 

Air  Gap,  10,500 

Field  Cores,  2,230 

Field  Yoke,  2,560 

Total,  1  9,  1  60  ampere-turns. 


104  DYNAMO   ELECTRIC  MACHINERY. 

Therefore  9580  ampere-turns  must  be  provided  on  each 
pole  in  order  to  set  up  the  required  flux.  This  is  true  only 
when  the  flux  distribution  through  each  section  is  uniform, 
which  condition  exists  when  no  current  flows  through  the 
armature,  that  is,  at  no  load.  Additional  ampere-turns  must 
be  provided  when  the  generator  delivers  energy  so  as  to 
neutralize  the  effects  of  demagnetization  and  distortion, 
which  will  be  discussed  in  Chapter  V. 

49.  Field  Coils.  -  In  a  shunt-wound  machine,  the  am- 
pere-turns necessary  for  excitation  are  obtained  by  a  rela- 
tively small  current  flowing  through  many  turns  of  wire, 
whereas  in  a  series-wound  machine  the  required  ampere- 
turns  are  obtained  by  sending  the  entire  current,  or  a  definite 
part  of  it,  through  but  a  few  turns  of  wire.  In  a  compound- 
wound  machine,  the  shunt  winding  supplies  the  ampere- 
turns  required  to  produce  the  definite  magnetic  flux  through 
the  armature  at  no  load,  and  the  series  winding  supplies  the 
additional  ampere-turns  necessary  for  full-load  operation. 

Knowing  the  necessary  ampere-turns  per  field  pole  at 
no  load,  the  size  or  cross-section  of  the  wire  to  be  used 
for  the  shunt-field  winding  may  be  calculated  as  follows  : 

Let  T8h  =  current  in  the  shunt-field  winding  in  amperes, 

E    =  terminal  voltage  of  machine  at  no  load, 
and      Erh  =  voltage  allowance  in  regulating  rheostat  ; 


then  the  resistance  of  each  shunt-field  coil  in  ohms  is 

E-Erh 


,, 


where  p  is  the  number  of  pairs  of  poles. 

The  temperature  rise  of  the  field  coils  under  full-load 
operation  should  not  exceed  50°  C.  above  the  usual  engine- 


FIELD    MAGNETS.  IO5 

room  temperature  of  25°  C.  At  a  temperature  of  75°  C. 
the  resistance  between  opposite  faces  of  an  inch  cube  of 

copper  is    >594  (i  +  0.0042  x  75)  =  0.825  microhms  (§  4.) 
2-54 

Representing  the  number  of  turns  on  one  shunt  coil  by 
nsh,  the  mean  length  of  a  turn  in  inches  by  Lsh,  and  the 
section  of  the  conductor  in  square  inches  by  Ash,  then  the 
resistance  of  each  shunt-field  coil  in  ohms  is 

o.S2$nshLsh  . 

~^T 

Equating  (i)  and  (2)  it  follows  that  the  conductor  cross- 
section  of  the  shunt  winding  is 


If  circular  conductors  are  to  be  employed  the  proper  size 
may  be  determined  by  reference  to  a  wire  table. 

As  an  example  on  the  foregoing,  calculate  the  section 
of  the  shunt-field  conductor  necessary  to  provide  9580 
ampere-turns  on  each  field  pole  of  the  35O-K.W.,  8-pole, 
5oo-volt  generator  of  §  48. 

Assume  15%  of  the  generator  voltage  to  be  taken 
up  by  the  adjusting  rheostat,  and  that  the  depth  of  the 
field  winding  is  3  inches.  The  mean  length  of  a  turn  is 
then  74  inches.  Therefore  the  conductor  area  of  the 
shunt  winding  is 

0.825  X  74  X  9580  X  8 

Ash  =  -  ~t  —  —  --  -~—K  -  =  o.on  sq.  in. 
(5oo-75)io6 

The  space  occupied  by  the  conductors  depends  upon  the 
number  of  poles,  voltage,  and  speed  of  the  machine,  and  upon 


106  DYNAMO  ELECTRIC   MACHINERY 


Fig.  66. 


c 


Fig.  67. 


FIELD   MAGNETS.  IO/ 

the  shape  of  the  conductor  section.  As  a  rule,  from  40% 
to  70%  of  the  total  cross-sectional  area  of  a  field  coil  is 
occupied  by  copper,  the  remainder  being  taken  up  by 
insulation.  The  ratio  of  the  total  copper  section  to  the 
gross  section  of  the  coil  is  called  the  space  factor  of  the 
coil. 

Having  determined  the  space  factor  of  a  proposed  shunt- 
field  winding,  the  length  of  the  coils  may  be  computed, 
and  the  available  space  for  the  series  winding,  if  any,  may 
be  obtained.  Then  the  calculation  of  the  number  of  turns 
and  the  size  of  conductor  on  the  series  winding  follows, 
the  method  of  procedure  being  similar  to  that  for  the 
shunt  winding. 

Shunt  coils  are  usually  made  of  cotton-insulated  round 
or  rectangular  conductors,  wound  on  metal  frames  or  on 
removable  molds  and  held  in  shape  by  paper  and  rope 
bands,  the  exterior  of  the  coils  being  coated  with  moisture- 
proof  insulating  varnish.  The  coils  of  series  machines 
and  the  series  coils  of  compound-wound  dynamos  are  very 
often  of  forged  copper  conductors  insulated  with  tape, 
the  individual  turns  being  separated  by  spacing  pieces. 
Typical  Westinghouse  series  and  shunt-field  coils  are 
shown  in  Fig.  66.  The  current  density  in  shunt  coils  of 
the  usual  construction  is  about  1000  amperes  per  square 
inch,  and  in  series  coils  it  may  be  20  %  greater  because 
of  the  superior  heat-radiating  facilities. 

A  completed  and  assembled  field  winding  of  a  250- 
K.W.,  2 50- volt,  compound- wound  generator  is  depicted 
in  Fig.  67,  in  which  are  shown  the  connections  of  the 
series  field  coils  at  the  end  of  the  machine  away  from 
the  commutator. 


108  DYNAMO   ELECTRIC   MACHINERY. 

PROBLEMS. 

1.  The  resistance  of  the  field  winding  of  a  I5-K.W.,  220- 
volt  shunt-wound  generator  is  60  ohms.     What  percentage  of 
the   full-load   power   output   is   the  power  consumed  in   field 
excitation  ? 

2.  A  series-wound  generator  supplies  current  to  five  arc 
lamps  connected  in  series,  each  taking  9.6  amperes  at  47  volts. 
The  resistance  of  the  field  winding  is  1.2  ohms,  and  the  drop  in 
the  external  circuit  is  15  volts.     Calculate  the  percentage  of  the 
total  power  expended  in  exciting  the  field  magnets. 

3.  The  flux   density  in  the  field-magnet  cores,  which    are 
6  inches  in  diameter,  of  a  bipolar  dynamo  is  80  kilomaxwells  per 
sq.  in.,  and  the  dispersion  coefficient  of  the  machine  as  deter- 
mined experimentally  is  1.35.     Calculate  the  magnitude  of  the 
flux  passing  through  the  armature. 

4.  Calculate  the  number  of  no-load  ampere-turns  to  be  pro- 
vided on  each  field  pole  so  that  a  flux  of  6  megamaxwells  per 
pole  may  be  sent  through  the  armature  of  a  ioo-K.W.,  6-pole, 
no-volt,  compound-wound  dynamo  having  the  following  con- 
stants and  dimensions : 

Armature  outside  diameter  =  32  in. 
Armature  internal  diameter  =  20  in. 
Armature  gross  length  =  9  in. 
Two  ventilating  ducts,  each  0.4  in.  wide. 
125  armature  slots,  each  i  in.  X  0.4  in. 
Armature  core  of  laminated  iron. 
Radial  length  of  air  gap  =  0.2  in. 
Dispersion  coefficient  =  1.20. 
Diameter  of  field  cores  =  9.5  in. 
Polar  span  =  70%. 
Field  structure  of  cast  steel. 
Internal  diameter  of  yoke  =  54  in. 
Outside  diameter  of  yoke  =  62  in. 
Yoke  width  =  10  in. 


PROBLEMS.  109 

5.  Allowing  20%  of  the  voltage  of  the  generator  of  Prob.  4 
to  be  lost  in  the  adjusting  rheostat,  determine  the  size  of  cir- 
cular copper  wire  to  be  used  in  the  shunt  winding.    The  outside 
diameter  of  the  field  coils  is  13  inches,  and  the  net  length  of 
the  winding  is  9  inches. 

6.  Calculate  the  total  resistance  and  number  of  turns  of  the 
shunt  winding  of  the  generator  of  the  two  foregoing  problems, 
the  space  factor  of  the  field  coils  being  0.60. 


110  DYNAMO   ELECTRIC   MACHINERY. 


CHAPTER   V. 

ARMATURE    REACTION.     COMMUTATION. 

50.  Armature  Reaction.  —  When  an  armature  revolves 
in  a  magnetic  field  an  electromotive  force  is  developed  in 
its  conductors,  and  this  is  available  at  the  brushes  of  the 
machine.  If,  therefore,  the  brushes  be  connected  through 
an  external  circuit,  a  current  will  flow  through  it,  and  the 
current  strength  will  depend  upon  the  resistance  of  the  cir- 
cuit. This  current,  in  flowing  through  the  armature  wind- 
ing, will  exert  a  magnetizing  action  on  the  core  independently 
of  the  field  magnets ;  thus  there  are  two  coexistent  mag- 
netic fields  with  directions  at  an  angle  to  each  other.  As 
the  resultant  field  differs  in  direction  from  that  caused 
only  by  the  current  in  the  field  coils,  it  is  frequently  said 
that  the  current  in  the  armature  conductors  causes  a  dis- 
tortion of  the  flux  in  the  magnetic  circuit,  and  this  effect 
of  the  armature  current  is  called  the  cross-magnetizing 
effect.  Because  of  cross-magnetization,  it  is  necessary,  in 
order  to  secure  good  commutation,  to  shift  the  position  of 
the  brushes  away  from  the  geometrical  median  line  be- 
tween radii  to  two  adjacent  poles.  A  shifting  of  brushes 
from  this  neutral  plane  results  in  a  weakening  of  the  mag- 
netic field,  due  to  the  armature  current  in  some  of  the 
conductors,  and  this  effect  is  called  the  demagnetizing 
effect.  These  two  effects  of  the  armature  current,  i.e.,  the 
cross-magnetizing  and  demagnetizing  effects,  when  consid- 
ered together,  are  called  armature  reaction  or  armature 
interference. 


ARMATURE    REACTION. 


Ill 


51.  Cross-Magnetizing  Effect  of  Armature  Current. — 
Consider  a  drum  armature  revolving  in  a  bipolar  magnetic 
field,  and  let  the  brushes  be  placed  midway  between  the 
poles,  that  is,  in  the  neu- 
tral plane.  When  the 
field  magnets  are  excited 
and  the  armature  is  run- 
ning on  open  circuit, 
the  magnetic  flux  which 
passes  through  the  arma- 
ture core  may  be  repre- 
sented by  dotted  lines  in 
Fig.  68,  and  it  is  seen 
to  be  quite  uniformly  distributed.  Upon  interrupting  the 
exciting  circuit,  and  sending  a  current  from  some  outside 

source  through  the  gener- 
ator armature,  the  resulting 
magnetic  field  may  be  de- 
picted as  in  Fig.  69.  This 
is  called  the  cross  flux 
because  its  axis  is  across 
that  of  the  main  field  flux. 
In  operation,  however,  both 


of  these  fields  exist  simul- 
taneously, and  the  resultant 
flux  through  the  armature 
is  obtained  by  combining  these  two  conditions  as  in  Fig.  70. 
The  flux  distribution  through  the  generator  field  poles  and 
armature  is  thus  non-uniform,  and  the  distortion  takes  place 
in  the  direction  of  rotation,  resulting  in  the  crowding  of 
lines  of  force  in  the  trailing  pole  tips.  This  distortion  of 
the  magnetic  flux  occasions  a  loss  in  the  operation  of  a 


OO 


Fig.  69. 


I  12 


DYNAMO   ELECTRIC   MACHINERY. 


generator  because  it  increases  the  reluctance  in  two  ways, 
—  (a)  by  saturating  the  iron  at  the  pole  pieces  and  thus 


Fig.  70. 


reducing  the  permeability,  and  (b)  by  lengthening  the  paths, 
both  in  air  and  in  iron,  that  the  lines  of  force  follow. 


Fig.  71. 


The  composition  of  the  two  apparent  magnetic  fluxes  in 
the  air-gap  of  a  bipolar  dynamo  is  illustrated  in  Fig.  71, 
which  shows  rectified  curves  of  magnetic  distribution  under 
the  pole  pieces  and  in  the  air-gap  of  the  machine,  ordinates 


ARMATURE    REACTION.  113 

of  the  curves  representing  flux  density.  Curve  a  represents 
the  symmetrical  flux  distribution  occasioned  by  the  current 
in  the  field  magnets,  and  curve  b  shows  the  flux  distribution 
in  the  air-gap  due  to  the  armature  current.  Adding  the 
orclinates  of  these  curves  yields  the  resultant  flux  dis- 
tribution actually  existing  in  the  air-gap  of  dynamos  under 
load  when  brushes  are  in  neutral  plane,  as  indicated  in 
curve  c. 

52.  Demagnetizing  Effect  of  Armature  Current. — The 
effect  of  the  cross  flux  is  to  produce  with  the  main  flux 
a  resultant  magnetic  field  in  the  air-gap  of  a  dynamo  the 
direction  of  which  is  inclined  to  the  direction  of  the  field 
flux,  as  shown  by  the  arrows  in  Fig.  70.  Since  the  brushes 
should  be  at  those  points  where  they  may  short-circuit  coils 
whose  conductors  begin  to  cut  magnetic  lines  of  force  in  a 
reversed  direction,  the  position  of  the  brushes  should  not  be 
midway  between  the  poles  (or  in  the  neutral  plane),  but  at 
right  angles  to  the  direction  of  the  resultant  flux.  Therefore 
the  plane  of  the  brushes  through  the  armature  axis,  known 
as  the  commutating  plane,  should  be  shifted  away  from  the 
neutral  plane  for  a  generator  in  the  direction  of  rotation. 
This  shifting  of  the  commutating  plane  causes  a  further 
distortion  of  the  flux,  necessitating  a  continued  shifting 
of  the  brushes  until  the  commutating  plane  is  displaced 
90  electrical  degrees  from  the  final  resultant  flux.  In 
the  foregoing,  the  armature  conductors  are  supposed  to 
be  connected  to  commutator  segments  lying  on  the  same 
radius. 

For  generators,  the  commutating  plane  is  in  advance  of 
the  neutral  plane,  and  for  motors,  behind  the  neutral  plane. 
The  angle  between  these  planes  is  termed  the  angle  of  brush 
lead  or  brush  lag. 


DYNAMO   ELECTRIC   MACHINERY. 


In  Fig.  72  is  shown  a  generator  armature  revolving  in  a 
bipolar  field,  the  brushes  being  given  a  forward  displacement 
of  6  degrees.  The  armature  conductors  may  be  divided  into 
two  separate  groups  by  the  two  vertical  lines  ab  passing 


Fig.  72.  % 

through  the  brush  contacts,  each  group  being  considered  as 
forming  a  number  of  complete  turns.  The  current  flowing 
in  the  turns  which  lie  outside  of'  these  lines  sets  up  the 
cross  flux  as  shown  in  §  51,  and  the  current  flowing  through 
the  turns  included  between  the  lines  ab  produces  a  mag- 
netomotive force  opposing  that  of  the  field-magnet  coils, 
and  thus  exerting  a  demagnetizing  effect  on  the  magnetic 
circuit.  The  product  of  the  number  of  turns  between  aa 
and  the  current  flowing  in  them  is  called  the  demagnetizing 
or  back  ampere-turns,  since  it  produces  a  weakening  of  the 
magnetic  field. 

The  current  flowing  in  the  armature  conductors  of  a 
multipolar  dynamo  similarly  exerts  distorting  and  demag- 
netizing effects  upon  the  magnetic  field,  and  the  armature 
turns  may  also  be  divided  into  two  belts,  namely  cross  turns 
and  back  turns. 


ARMATURE   REACTION.  115 

53.  Compensation  for  Armature  Reaction.  —  To  neutral- 
ize the  effects  of  cross-magnetization  and  demagnetization 
produced  by  the  current  flowing  in  dynamo  armatures,  it  is 
necessary  to  provide  additional  magnetomotive  force  by  in- 
creasing the  ampere-turns  of  the  field-magnet  coils.  Com- 
pensation for  demagnetization  is  easily  calculated,  since  the 
number  of  back  turns  times  the  current  flowing  through  them 
at  any  load,  multiplied  by  the  dispersion  coefficient  at  that 
load,  gives  the  additional  number  of  ampere-turns  necessary 
at  that  load  for  compensation.  The  additional  field  ampere- 
turns  per  pole  necessary  to  overcome  the  demagnetizing 
effect  of  the  armature  current  at  full  load  may  be  repre- 
sented by 


where     6  =  brush  lead  in  electrical  degrees  (angular  distance 
between   centers    of    like-named    poles  =  360 
electrical  degrees), 
5  =  number  of  armature  conductors  in   series  be- 

tween two  brushes  (§  39), 
/=  full-load  current  of   dynamo,  leaving  or  enter- 

ing at  the  brushes, 
and        d  =  dispersion  coefficient. 

The  additional  field  ampere-turns  necessary  to  neutralize 
the  distortional  effect  of  the  armature  current  are  difficult  to 
determine,  but  the  following  method  of  calculation,  based 
upon  experimental  data,  yields  a  good  estimation.  The 
ampere-turns  of  the  armature  producing  the  cross-magneti- 
zation, being  due  to  the  current  in  the  cross  turns  only,  are 

D'  =  (i-24-\—I     per  pole. 
V         i8o/4 


n6 


DYNAMO   ELECTRIC   MACHINERY. 


The  ratios  of  this  quantity  D'  to  the  number  of  ampere- 
turns  per  pole  on  the  field  magnets  required  for  compensa- 
tion of  distortion,  (nl)cm,  are  the  ordinates  of  Fig.  73 ;  and 
the  abscissae  represent  the  no-load  ampere-turns  per  pole 
on  the  field-magnet  coils.  This  ratio,  for  values  of  D'  be- 
tween 1000  (lower  curve)  and  10,000  (upper  curve),  lies 
between  the  two  curves,  its  magnitude  being  determined  by 
interpolation.  Knowing  the  no-load  field  ampere-turns,  and 
having  calculated  the  value  of  D'  from  the  above  equation 
at  full-load  current,  the  number  of  ampere-turns  per  field 
spool,  (»/)m>  necessary  to  compensate  for  distortion  due  to 


V; 


4  8 

NO-LOAD  FIELD 
KILO-AMPERE-TURNS 


16 


Fig.  73- 

cross-magnetization  at  this  load,  may  therefore  be  obtained 
from  Fig.  73. 

As  a  numerical  example  of  the  foregoing,  calculate  the 
field  ampere-turns  per  pole  necessary  to  compensate  for 
armature  reaction  at  full  load  in  the  35O-K.W.,  5OO-volt 
generator  of  §48,  when  the  angle  of  brush  lead  is  5  degrees. 


ARMATURE    REACTION.  1  1/ 

The  armature  has  an  eight-circuit  winding  embedded  in 
240  slots,  there  being  6  conductors  per  slot.  The  number 
of  armature  conductors  in  series  between  positive  and  nega- 

tive brushes  is  240  X  6 

o  =  -  =  loO. 

2X4 

The  full-load  current  of  the  dynamo  is 
=  35Q  X  looo  =         affl 

500 

Therefore  the  field  ampere-turns  necessary  to  neutralize  de- 
magnetization at  full  load  are. 


(•")*.  -xx  700x1.15  =  500. 


The  ampere-turns  of  the  armature  producing  cross-magneti- 
zation are 


The  field  ampere-turns  per  pole  at  no  load  =  9580  (§48). 
With  this  value  as  abscissa,  the  ordinate  corresponding  to 
would  be  2.8.  Hence 


from  which  the  ampere-turns  per  field  pole  required  for 
overcoming  effect  of  distortion  at   full  load  are  (nl)cm  = 


The  total  ampere-turns,  then,  to  be  provided  on  each  field 
pole  are  9580  +  500  +  2650=  12,730.  When  a  dynamo  de- 
livers current  there  is  a  resistance  drop  in  the  machine 
itself,  so  that  the  actual  induced  voltage  must  be  somewhat 
larger  than  the  rated  terminal  voltage.  The  additional 


u8 


DYNAMO   ELECTRIC   MACHINERY. 


ampere-turns  required  for  producing  the  greater  magnetic 
flux  necessary  to  obtain  the  higher  internal  voltage  have 
not  been  considered  in  the  foregoing. 

54.  Devices  for  Reducing  Armature  Reaction.  —  The 
distortion  of  the  magnetic  field  may  be  diminished  by 
lengthening  the  air-gap  and  working  the  armature  teeth  at 
high  flux  densities,  thereby  increasing  the  reluctance  of  the 
path  of  the  cross  flux.  This,  however,  also  increases  the 
reluctance  of  the  main  flux  path  and  necessitates  the  pro- 
vision of  a  greater  number  of  ampere-turns  on  the  field- 
magnet  coils. 


Fig.  74- 

Magnetic  field  distortion  may  be  reduced  by  slotting  the 
field  poles  longitudinally.  This  considerably  increases  the 
reluctance  of  the  path  of  the  cross  flux  by  introducing  in  it 
an  air-gap,  whereas  the  reluctance  of  the  main  flux  path  is 
very  slightly  altered.  Fig.  74  shows  a  Lundell  split -pole 


ARMATURE    REACTION.  1 19 

type  of  generator  in  section  and  illustrates  the  construction 
of  the  pole  piece.  The  magnetic  flux  which  enters  the 
pole  piece  divides  between  the  two  paths  a  and  b.  Owing, 
however,  to  the  greater  span  covered  by  the  shoe  belonging 
to  the  part  marked  b,  the  magnetic  reluctance  of  that  part 
is  much  smaller  than  that  of  the  part  marked  a.  As  a  result, 
the  flux  does  not  divide  itself  equally  between  the  two  paths. 
The  part  of  the  pole  piece  marked  b  under  increasing  exci- 
tation becomes  saturated  before  the  part  marked  a.  At 
normal  excitation,  the  flux  density  at  b  is  above  16,000  lines 
per  square  centimeter,  while  the  flux  density  in  a  is  but 
about  10,000  lines  per  square  centimeter.  In  other  words, 
b  is  well  saturated,  while  the  magnetization  of  a  is  still  below 
the  knee  of  the  magnetization  curve.  This  saturation  of 
half  of  the  pole  piece  is  effective  in  preventing  a  skewing 
of  the  field  by  the  cross  turns  as  the  load  on  the  machine 
increases.  This  is  shown  in  the  flux  distribution  curve  of 
Fig.  75  >  wherein  the  arrows  show  the  direction  of  rotation 


Fig.  75- 

of  the  armature.  The  dotted  line  represents  the  distribu- 
tion at  no  load,  and  the  heavy  line  the  distribution  at  full 
load.  This  small  distorting  effect  of  the  cross  turns  permits 
the  employment  of  a  small  air-gap  without  causing  serious 
sparking. 


120  DYNAMO   ELECTRIC    MACHINERY. 

Ryan  compensates  for  the  magnetizing  effects  of  the 
armature  winding  by  surrounding  the  armature  with  a  sta- 
tionary winding,  which  passes  through  perforations  in  the 
pole  faces.  These  stationary  windings  carry  the  whole 
current  of  the  machine,  being  connected  in  series  with  the 
external  circuit.  The  current  in  these  windings  produces 
a  magnetomotive  force  equal  and  opposite  to  that  due  to 
the  armature  current.  The  number  of  ampere-turns  on  the 
compensating  winding  is  about  one  and  one-quarter  times 
the  armature  cross  ampere-turns.  This  arrangement  is  not 
much  used  in  direct-current  practice  because  of  construc- 
tive difficulties  and  increased  cost. 

Distortion  of  magnetic  field  is  further  decreased  by  prop- 
erly shaping  the  pole  pieces.  The  distribution  of  flux 
should  be  such  that  an  armature  coil  at  first  enters  a  weak 
field  and  then  gradually  comes  to  the  strongest  part.  If 
the  lines  of  force  are  allowed  to  crowd  into  the  trailing-pole 
tips,  this  gradual  transition  is  impossible.  If  the  horns  are 
farther  from  the  armature  surface  than  the  body  of  the  pole 
face,  then  the  air-gap,  and  consequently  the  reluctance  at 
the  horns,  is  increased,  and  the  lines  of  force  are  dis- 
tributed more  symmetrically.  The  poles  may  have  cham- 
fered corners  or  be  non-concentric  with  the  armature, 
the  radius  of  the  latter  being  less  than  that  of  the  pole 
faces. 

The  use  of  auxiliary  poles  between  the  main  field  poles 
of  dynamos  is  also  effective  in  reducing  armature  reaction. 
The  coils  on  the  auxiliary  poles  are  connected  in  series 
with  the  armature,  and  the  entire  current,  or  a  definite  part 
of  it,  traverses  the  auxiliary  winding.  This  arrangement 
yields  perfect  commutation  at  all  loads  for  various  speeds 
with  a  definite  setting  of  the  brushes.  The  field  structure 


COMMUTATION. 


121 


of  a   dynamo   with   auxiliary  poles   manufactured   by   the 
Electro-Dynamic  Company  is  shown  in  Fig.  76. 


Fig.  76. 

55.  Commutation. — The  electromotive  force  induced  in 
the  armature  conductors  of  practically  all  direct-current 
generators  is  alternating,  and  in  order  to  obtain  a  unidirec- 
tional E.M.F.  at  the  terminals  of  the  machine  it  is  necessary 
to  reverse  the  connections  at  the  moment  the  induced  elec- 
tromotive force  changes  its  direction.  This  process  is  called 
commutation,  and  is  accomplished  by  means  of  a  commutator, 
whose  segments  are  connected  to  the  armature  windings, 
and  brushes  which  collect  the  current  from  the  commu- 
tator. During  the  process  of  commutation,  the  armature 
conductors  connected  to  the  commutator  segments  covered 


122  DYNAMO   ELECTRIC   MACHINERY. 

by  a  brush  are  momentarily  short-circuited  In  this  inter- 
val of  time,  the  current  flowing  in  a  coil  must  be  changed 
from  a  maximum  value  in  one  direction  to  zero,  and  from 
zero  to  a  maximum  value  in  the  other  direction. 

The  current  in  an  armature  coil,  at  the  beginning  of  a 
short-circuit  by  a  brush,  is  responsible  for  the  existence  of 
a  definite  amount  of  magnetic  flux  which  is  linked  with  the 
turns  of  the  coil.  This  flux  would  disappear  if  the  current 
were  suppressed,  or  would  be  built  up  in  a  reversed  direc- 
tion upon  the  reversal  of  direction  of  the  current.  The 
product  of  this  flux  and  the  number  of  turns  in  the  coil 
divided  by  the  io8  times  the  current  in  amperes  flowing 
through  the  coil  at  the  beginning  of  short-circuit,  gives  a 
quantity  which  may  be  termed  the  commutation  self-induc- 
tance of  the  coil.  Representing  this  quantity  by  L,  the 
number  of  turns  of  the  coil  by  n,  and  the  number  of  mag- 
netic lines  of  force  accompanying  a  current  of  /t  amperes 
flowing  in  the  coil  by  4>,  then  the  commutation  self-induc- 
tance is 

n$    , 

L  =  -  -  -  henrys. 
8 


The  magnetic  flux  surrounding  a  coil  carrying  a  current 
represents  an  amount  of  energy  the  magnitude  of  which  is 
equal  to  \LI*.  This  energy  must  be  disposed  of  and  an 
equal  amount  oppositely  directed  must  exist  before  the  coil  is 
in  proper  condition  to  be  transferred  from  one  side  of  the 
armature  to  the  other  at  the  brush.  The  magnetic  energy 
may  be  dissipated  either  by  the  introduction  of  resistance 
to  reduce  the  current  flow  or  by  a  counter-electromotive 
force,  which  may  be  obtained  by  so  placing  the  brushes 
that  the  short-circuited  coil  will  cut  some  of  the  flux  from 
that  pole  corner  of  the  field  magnet  toward  which  the  coil 


COMMUTATION.  123 

is  moving.  That  is,  the  commutating  plane  is  shifted  so 
that,  during  the  time  a  coil  is  short-circuited  by  a  brush, 
the  coil  will  traverse  a  magnetic  field  of  opposite  direction 
having  such  intensity  as  to  produce  an  E.M.F.  in  it  sufficient 
to  reverse  the  direction  of  the  initial  current.  If,  during 
the  time  of  short-circuit,  the  intensity  of  the  current  after 
reversal  be  identical  with  its  original  intensity,  then  spark- 
less  commutation  will  ensue  with  this  particular  brush  set- 
ting. At  any  other  position  of  the  brushes,  the  magnitude 
of  the  induced  E.M.F.  will  be  such  as  to  result  in  a  larger 
or  smaller  current  value  in  the  coil  after  reversal  than  its 
initial  value,  consequently  sparking  will  occur  during  the 
process  of  commutation  at  such  positions  of  the  brushes. 
The  greater  the  current  output  of  a  machine  the  greater 
must  be  the  induced  E.M.F.  to  effect  current  reversal,  and 
consequently  the  field  in  which  the  coil  is  situated  during 
commutation  must  be  more  intense,  and  therefore  the  brush 
position  should  change  with  the  load.  The  present  practice, 
however,  is  to  have  a  definite  brush  position  which  is  the 
same  at  all  loads  upon  the  machine.  Therefore  the  com- 
mutating plane  should  be  shifted  forward  in  generators  until 
no  sparking  occurs  in  the  final  position  at  full-load  and  also 
at  no-load  operation. 

There  are  thus  two  E.M.F. 's  active  in  producing  a  current 
through  the  short-circuited  armature  coil:  first,  the  electro- 
motive force  induced  in  the  coil  by  cutting  the  magnetic 
lines  of  force  set  up  by  the  currents  in  the  field  magnet 
coils,  and  second,  the  E.M.F.  occasioned  by  the  varying 
flux  due  to  the  changing  current  in  the  coil.  The  latter 
E.M.F.  is  independent  of  any  action  of  the  field-magnets, 
and  is  called  an  electromotive  force  of  self-induction  or  re- 
actance voltage,  and  it  tends  to  prevent  any  increase  or  de- 


124  DYNAMO   ELECTRIC   MACHINERY. 

crease  in  the  strength  of  the  current  flowing.  Sparkless 
commutation  is  dependent  upon  the  magnitude  of  the  re- 
sultant E.M.F.  of  these  two  opposing  components.  Were 
the  resultant  E.M.F.  zero  at  every  instant,  perfect  commu- 
tation might  be  obtained.  But  this  ideal  condition  cannot 
be  realized,  since  the  component  electromotive  forces  con- 
stitute different  time  functions.  The  induced  E.M.F.  de- 
pends upon  the  intensity  of  the  field  and  speed  of  the 
armature,  whereas  the  E.M.F.  of  self-induction  depends 
upon  the  rate  of  current  change  in  the  coil.  An  approach 
to  perfect  commutation,  therefore,  is  obtained  by  an  adjust- 
ment of  conditions  whereby  a  number  of  instantaneous 
values  of  these  opposing  E.M.F.'s  are  equal  during  the 
time  the  coil  is  short-circuited  by  the  brush  or  while  it 
undergoes  commutation. 

The  current  reversal  in  the  armature  coils  is  accelerated 
by  the  use  of  high-resistance  brushes  because  the  initial 
current  is  more  quickly  reduced  to  zero.  Consider  a  coil 
of  low  resistance  to  be  short-circuited  by  a  high-resistance 
carbon  brush,  as  shown  in  Fig.  77,  the  brush  having  the 
same  width  as  the  commutator  seg- 
ments. Let  z'j  and  i2  be  the  currents 
flowing  in  the  taps  to  segments  I 
and  2  respectively,  and  let  /t  be  the 
current  flowing  through  each  arma- 
R  ture  path  between  brushes.  At  the 

f\  /*\  r\  r\  r\  r\  /~\ 

instant  when  commutator  segment 
i  is  completely  under  a  brush,  the 
currents  from  both  sides  of  the 

armature  unite  and  pass  through  the  corresponding  tap; 

then 

i1  =  2  7t     and     i2  =  o. 


COMMUTATION.  125 

A  moment  later,  the  brush  will  be  in  contact  with  both 
segments  ;  then,  if  /'  be  the  short-circuited  current, 

it  =  /!  +  /'     and     i2  =  /,  -  /'. 

In  this  position,  the  high  transition  resistance  between  the 
brush  and  commutator  will  be  less  at  segment  I  than  that  at 
segment  2,  because  of  the  greater  contact  area  of  the  for- 
mer ;  therefore  the  voltage  drop  across  the  contact  at  seg- 
ment 2  will  be  greater  than  that  at  the  other  segment. 
Because  of  this  difference  of  voltage  drops  across  the  two 
segments,  there  is  a  tendency  for  a  current  to  flow  in  a 
direction  opposite  to  the  direction  of  z'r  This  condition 
results  in  quicker  reversal  of  the  current. 

Continuing  the  cycle  of  commutation,  a  little  later  the 
brush  contact  area  on  both  segments  will  be  the  same  ; 
then,  if  the  short-circuit  current  be  zero, 


The  next  moment  the  direction  of  current  /'  will  be  re- 
versed, and 

i^=I^-T     and     *a  =/!  +  /'. 

Finally,  -when  the  brush  rests  only  on  segment  2, 
^  =  o     and     i2  =  2  7r 

The  current  density  in  the  brushes  varies  considerably  at 
different  parts  of  it,  and  is  not  proportional  to  the  E.M.F. 
because  of  the  exceedingly  variable  transition  resistance 
between  brushes  and  commutator.  This  increases  very 
rapidly  if  there  be  even  minute  sparking  under  the  brushes. 

56.  Time  of  Commutation.  —  The  time  interval  during 
which  the  current  changes  from  .a  maximum  value  in  one 
direction  to  an  equal  value  in  the  opposite  direction  is  the 


126  DYNAMO   ELECTRIC   MACHINERY. 

time  that  elapses  from  the  instant  that  one  commutator 
segment  reaches  a  brush  to  the  instant  the  preceding  seg- 
ment emerges  from  the  other  side  of  the  brush.  This  time 
is  evidently  dependent  upon  the  speed  of  the  commutator 
and  upon  the  width  of  the  brushes,  and  is  the  time  required 
for  the  strip  of  insulation  between  two  successive  segments 
to  pass  under  the  brush.  If  wb  represent  the  breadth  of 
the  brush  in  inches,  and  m  the  thickness  of  mica  between 
adjacent  commutator  segments  in  inches,  then  the  time  of 
the  short-circuit  is 

(wb  —  m) 
tc  =  -  -  -  seconds, 

v 

where  v  is  the  peripheral  velocity  of  the  commutator  in 
inches  per  second.  Representing  the  commutator  diameter 
in  inches  by  Dc,  and  the  number  of  revolutions  per  minute 
of  the  armature  by  V,  then 


60 

Therefore 

_  60  (iVb  —  m) 
nDcV 

The  reciprocal  of  this  time  gives  the  number  of  commu- 
tations per  second,  or  what  is  termed  the  frequency  of  com- 
mutation. The  frequencies  found  in  practice  lie  between 
200  and  800  per  second.  The  breadth  of  a  brush  may  be 
equal  to  the  width  of  a  commutator  segment,  but  usually 
the  brush  is  sufficiently  broad  to  bridge  over  several  seg- 
ments. For  a  definite  brush  width,  the  frequency  of  com- 
mutation is  much  higher  for  multiplex  windings  than  for 
simplex  windings. 


COMMUTATION. 


127 


Let  +  /!  be  the  current  flowing  in  a  coil  just  entering 
under  a  brush.  After  the  time  t&  the  current  value  in  that 
coil  for  perfect  commutation  should  be  —  7r  What  the  in- 
stantaneous values  of  the  current  during  this  short  interval 
of  time  tc  will  be,  or  how  they  may  vary  in  a  particular 
machine  under  certain  conditions,  cannot  be  foretold,  yet 
Fig.  78  shows  a  possible  time  variation  of  the  current  in  a 


Fig.  78 


Fig.  79- 


coil  undergoing  commutation.  It  represents  a  sinusoidal 
current  change.  Numerous  curves,  such  as  Fig.  79,  have 
been  obtained  showing  the  actual  current  variation  during 
the  period  of  short-circuit  based  upon  experimental  obser- 
vations, but  such  curves  differ  widely  among  themselves, 
and  consequently  no  one  of  them  may  be  taken  to  represent 
the  general  conditions  occurring  during  commutation. 

57.  Calculation  of  Reactance  Voltage.  —  The  reactance 
voltage  of  a  short-circuited  armature  coil  is  occasioned  by 
the  varying  flux  which  surrounds  that  coil  when  the  current 
in  it  is  changing.  Its  value  depends  upon  the  time  rate 


128  DYNAMO   ELECTRIC   MACHINERY. 

of  current  change,  and  its  instantaneous  value  may  be  ex- 
pressed as 


where  L  is  the  commutation  self-inductance  of  the  coil  and 
/'  is  the  value  of  the  current  at  the  instant  /  seconds  after 
the  beginning  of  the  short-circuit.  It  cannot  be  predeter- 
mined exactly  how  the  successive  values  of  /'  are  related  to 
each  other  during  this  interval,  and  it  becomes  necessary, 
in  order  to  estimate  the  reactance  voltage,  to  assume  that 
the  values  of  the  short-circuit  current  follow  some  simple 
law.  It  is  usual  to  assume  a  simple  harmonic  variation,  as 
shown  between  the  dotted  lines  of-  Fig.  78,  in  which  case 
the  instantaneous  value  of  the  current  may  be  expressed  as 


/'    =   II  COS   2  7T  — I, 

2  //^ 


where  7t  is  the  current  per  armature  path,  and  tc  is  the 
time  of  short-circuit  (then 
second).     By  substitution 


time  of  short-circuit  (there  are  -  -  complete  variations  per 


<8 

whence 

"  —  sin 


r<  f  T  T 

E/  =£A-  si 

tc 


The  maximum  value  of  Es  occurs  when  —  is  90°  ;  then  the 


reactance  voltage  of  the  short-circuited  coil  is 


(0 


COMMUTATION. 


129 


The  time  of  commutation,  /c,  was  obtained  in  the  foregoing 
article  ;  thus  for  the  determination  of  the  reactance  voltage 
there  still  remains  the  calculation  of  the  self-inductance  of 
the  coil. 

To  calculate  the  magnitude  of  L  for  a  short-circuited 
armature  coil  or  element,  consider  each  coil  side  to  lie  in  a 
slot,  a  typical  form  of  which  is  shown  in  Fig.  80.  The  flux, 


Fig.  80. 

which  is  due  to  the  current  flowing  through  the  n  turns  of 
this  element,  may  take  a  number  of  paths  across  the  slot, 
as  shown  by  the  numbered  lines.  Some  lines  of  force  also 
encircle  the  coil  where  it  projects  beyond  the  slots.  The 
total  inductance  of  a  coil  will  be  the  sum  of  the  inductances 
due  to  the  flux  through  these  various  paths  linking  with  the 
turns  of  the  coil.  All  dimensions  shown  in  the  diagram 
will  be  expressed  in  inches. 

Consider  an  element  of  the  conductors  in   the  slot  dx 


130  DYNAMO   ELECTRIC   MACHINERY. 

wide  and  at  a  distance  x  from  the  bottom  of  the  slot.  The 
magnetomotive  force  which  produces  the  flux  in  this  element, 

due  to  the  current  7t  amperes  flowing  in  -of  the  n  con- 
ductors of  this  coil  side,  is 

M.M.F  =  4  n(*  n\  -^-gilberts. 
\a     I  10 

Since  the  permeability  of  the  iron  is  very  much  greater  than 
that  of  the  air,  the  reluctance  of  the  iron  portion  of  the  flux 
paths  may  be  neglected.  Then  the  reluctance  of  the  ele- 

mentary path  through  the  coil  itself  is  d(&.  =   -  —  - 

2.54  ladx 

oersteds,  where  la  is  the  gross  axial  length  of  the  armature 
core  in  inches.  Hence  the  flux  through  this  small  area  in 
maxwells  is 

x      L 
4?i:-n—L 
,  ,  a     10   _  471  2.54      nljaxdx 

ivs  i  o  aw 


2. 

These  lines  of  force  are  linked  with  -  n  turns,   and    there- 

a 

fore  the  elementary  inductance  in  henry  s,  being   io8  times 
the  number  of  linkages  per  ampere,  is 


(^-4*2.54 


Integrating  over  the  full  width  of  the  coil,  the  inductance 
due  to  the  flux  through  path  i  in  henrys  is 


wsio9 


Above  the  upper  surface  of  the  conductors,  the  magne- 
tomotive force  is  constant,  and  the  lines  of  force  through  the 


COMMUTATION.  131 

upper  portions  of  the  slot  are  linked  with  all  the  conductors 
of  the  coil-side.    The  reluctance  of  path  2  is *—-  and  the 

M.M.F.  sending  the  flux  through  it  is  <\nn— -  ;  therefore 

the  flux  is 

47:2.154     nljab 


Q   =  H^^OS  B  ^M^  maxwells. 
10  ws 

The  inductance  due  to  this  flux  is 


Similarly  the  inductances  due  to  the  magnetic  flux  through 
paths  3  and  4  linking  with  the  n  turns  of  the  coil  are 


T  ^  Q 

L*  =  63'8 

and 


For  two  surfaces,  WQ  inches  apart,  in  the  same  plane, 
the  paths  of  the  magnetic  lines  of  force  may  be  taken  as 
quadrants  joined  by  straight  lines  of  length  WQ.  Represent- 
ing the  width  of  a  tooth  at  the  air-gap  by  t^  and  consider- 
ing only  the  flux  outside  the  slot  between  two  adjacent  teeth, 
then  the  reluctance  of  the  mean  path  5  is 


&r  =  -  ;  —  oersteds. 
2.54  U, 

There  is  additional  flux,  also  occasioned  by  the  current  in 
the  coil  under  consideration,  which  passes  through  adjacent 
teeth  up  to  the  limit  of  the  interpolar  gap.  Because  of  its 
lesser  influence,  it  will  here  be  neglected.  It  is  approxi- 


132  DYNAMO   ELECTRIC   MACHINERY. 

mately  compensated  for  by  the  overestimation  of  the  induc- 
tance of  the  coil  as  thus  far  calculated,  due  to  neglecting 
the  reduction  of  effective  iron  area  occasioned  by  the  pres- 
ence of  air  ducts  and  insulation  between  laminations.  The 
flux  passing  through  path  5  is 


hence  the  resulting  inductance  is 
L*=  31.9; 


Therefore  the  total  inductance  of  the  embedded  portion  of 
one,  coil-side,  being  the  sum  of  the  terms  L^  to  L^  in  hen- 
rys,  is 

Le=  10.63  H'-  +  --  +  +  --lO-.     (2) 


To  obtain  the  inductance  of  a  complete  winding  element, 
i.e.,  two  coil-sides,  twice  this  value  Le  must  be  taken,  and 
to  it  must  be  added  the  inductance  of  the  end-connections, 
or  parts  of  the  coil  extending  beyond  the  slots.  It  is  usual 
to  assume  a  magnetic  flux  of  2  maxwells  per  ampere-inch 
of  conductor  as  linking  with  the  exposed  portion  of  a  wind- 
ing element.  The  length  of  the  end  connection  at  one  end 
of  the  armature  core  in  machines  of  the  usual  type  may  be 
taken  as  1.5  times  the  pole  pitch,  or  1.5^  inches.  The 
inductance  of  the  "free"  portion  of  an  armature  coil  of 
«  turns  would  then  be 


COMMUTATION. 


133 


Generally,  two  or  more  armature  coils  are  simultaneously 
undergoing  commutation,  and  since  the  coil-sides  of  two  or 
more  elements  are  usually  in  the  same  or  adjacent  slots, 
there  is  a  mutual  inductive  action  between  them.  In  Fig. 
8 1,  one  coil-side  of  the  winding  element,  short-circuited  by 


11 


Fig.  81. 

the  positive  brush,  lies  in  the  same  slot  with  one  coil-side 
of  the  element  short-circuited  by  the  negative  brush.  The 
mutual  inductance  with  such  an  arrangement  is  very  nearly 
equal  to  the  inductance  of  the  embedded  portion  of  a  coil. 
Thus  little  error  will  be  introduced  by  taking  double  the 
value  of  Let  as  previously  calculated,  to  include  the  effect 
of  mutual  inductance.  Since  the  end  connections  of  the 
two  coils  of  Fig.  8 1  do  not  coincide  but  are  widely  sepa- 
rated, no  mutual  inductive  action  between  these  portions  of 
the  winding  elements  need  be  considered. 

The  total  inductance  of  a  short-circuited  coil  is  therefore 

L  =  4Le  +  Lf.  (4) 

The  values  of  tc,  /t,  and  L  now  being  known,  the  reactance 


134  DYNAMO   ELECTRIC   MACHINERY. 

voltage  of  a  coil  undergoing  commutation  may  be  deter- 
mined from  equation  (i). 

In  dynamos  having  lap-wound  armatures  the  above  value 
of  L  would  be  that  for  a  coil  between  two  adjacent  com- 
mutator segments,  or,  as  sometimes  stated,  the  inductance 
per  segment.  In  dynamos  having  wave-wound  armatures 
with  only  two  brushes,  the  foregoing  value  of  coil  induc- 
tance is  that  for  one  winding  element.  For  one  coil  of 
this  type  of  armature  with  p  elements  terminating  at  two 
successive  commutator  segments,  the  inductance  would 
be  /  times  as  great.  Consequently  the  employment  of  as 
many  brushes  as  there  are  poles  is  desirable  from  the 
commutation  viewpoint. 

A  quick  method  of  estimating  the  inductance  of  the 
embedded  portion  of  an  armature  element,  due  to  Hobart, 
is  based  upon  the  assumption  that  a  flux  of  10  maxwells 
surrounds  each  inch  of  conductor  length  per  ampere  of 
current  which  flows  through  it.  Then 

(5) 


where  ln  is  the  net  axial  length  of  the  armature  core. 
Combining  with  equation  (3),  the  total  inductance  of  a 
short-circuited  coil  in  henrys  is 

L  =  w2(40/w  +  6^)io-8-  (6) 

Except  for  the  large  slow-speed  dynamos,  the  reactance 
voltage  should  not  exceed  2  volts  per  segment  in  order  to 
obtain  fair  commutation.  For  these  large  machines  the 
value  of  the  reactance  voltage  may  reach  5  volts,  but  the 
aim  of  the  designer  is  to  obtain  a  lower  value. 

58.  Conditions  for  Good  Commutation.  —  There  are  two 
E.M.F.'s  active  in  producing  a  current  through  an  arma- 


COMMUTATION.  135 

ture  coil  undergoing  commutation,  namely,  (i)  the  electro- 

motive force  of  self-induction,  which  is  equal  to  —  L  —  , 

at 

and  (2)  the  electromotive  force  induced  in  the  coil  due  to 
its  cutting  lines  of  force,  the  value  of  which  may  be  ex- 
pressed as  some  function  of  the  time,  or  f(t).  To  com- 
plete the  electromotive  force  equation  of  a  short-circuited 
coil,  the  various  resistance  drops  must  be  inserted.  Let 
R,  Fig.  77,  be  the  resistance  of  the  coil  undergoing  com- 
mutation, and  Rc  be  the  resistance  of  each  connection  tap 
to  the  commutator.  Then  the  resistance  drop  across  two 
adjacent  segments  (§55)  is 

PR  +  I.RC  +  i2Rc  =  PR  +  Rc  [I,  +  /'  +  A  -  P]  =  I'R  +  2  I.Rc. 
To  include  the  voltage  drop  from  the  brushes  to  the 
commutator,  let  R^  be  the  transition  resistance  of  one  set 
of  brushes,  having  a  breadth  equal  to  the  width  of  a  com- 
mutator segment,  when  resting  on  only  one  segment.  At 
the  time  t  seconds  after  the  beginning  of  the  short-circuit, 

the  transition  resistance  of  segment  i,  Fig.  77,  is 


tc  —  t 

and  that  of  segment  2  is  —  R^.      The  corresponding  resist- 
ance drops  are  respectively 
K  • 

K.I* 
e  ~ 

and 

c  ^c       ' 


The  complete  E.M.F.  equation  of  a  coil  undergoing  spark- 
less  commutation  may  then  be  expressed  as 


o. 


136  DYNAMO   ELECTRIC   MACHINERY. 

An  analysis    of  this  equation  has    shown  that  for  spark- 

r> 

less  commutation  in  the  neutral  plane  -1  tc  must  be  equal 

J^s 

to  or  greater  than  unity.  This  implies  that  the  transition  re- 
sistance of  brushes  should  be  great,  that  the  inductance  of 
the  short-circuited  coil  should  be  small,  and  that  the  time 
of  commutation  be  comparatively  large.  As  the  width  of 
the  brushes  is  generally  such  as  to  short-circuit  several  coils 
simultaneously,  the  above  general  equation  must  be  modi- 

r> 

fied  accordingly.      However,  -1  tc  remains  practically  un- 

J^s 

altered,  since  both  R1  and  L  are  multiplied  by  the  number 
of  coils  simultaneously  short-circuited  by  one  brush. 

Because  of  the  great  transition  resistance  between  cop- 
per commutator  segments  and  carbon  brushes,  these  are 
more  generally  used  than  copper  brushes,  although  the  latter 
find  application  in  high-speed  turbo-generators  as  well  as 
in  low-voltage  generators  or  motors.  For  a  fixed  brush 
position  at  all  loads  there  is  a  tendency  to  spark  when  the 
machine  is  subjected  to  wide  variations  of  load,  but  the  use 
of  carbon  brushes  counteracts  this  tendency  to  a  great  extent 
because  of  their  high  transition  resistance.  With  copper 
brushes,  a  fixed  brush  position  for  all  loads  can  rarely  be 

r> 

attained.     The  value  of  -1  tc  usually  exceeds  2  with  car- 

JL/ 

bon  brushes,  but  may  occasionally  fall  below  J  for  copper 
brushes. 

As  the  inductance  of  a  short-circuited  coil  depends  upon 
the  square  of  the  number  of  turns,  it  is  desirable  to  have 
few  turns  per  coil,  so  that  the  reactance  voltage  may  be  low. 
Good  practice  limits  the  number  of  turns  of  a  coil  in  moder- 
ate-sized machines  to  2  or  3.  An  inspection  of  equation  (2) 


COMMUTATION.  137 

of  §  57  shows  the  desirability  of  having  the  axial  length  of 
armature  core  small  so  as  to  reduce  Le ;  this  implies  that  the 
armature  diameter  be  large  for  the  same  output.  Large 
diameters  permit  of  the  employment  of  large  commutators 
having  many  segments,  consequently  there  will  be  fewer 
armature  turns  per  segment  than  would  be  possible  with 
smaller  commutators  having  bars  of  equal  width ;  this  con- 
dition, as  already  stated,  is  conducive  to  a  lower  reactance 
voltage. 

The  time  of  commutation  could  be  raised  by  increasing 
the  width  of  the  brush,  but  if  the  brush  bridges  over  more 
than  a  few  segments,  the  inductance  of  each  short-circuited 
coil  will  be  much  greater  because  of  its  turns  linking  with 
lines  of  force  produced  by  the  current  in  other  coils  as  well. 
A  further  limitation  to  increasing  the  time  of  short-circuit 
by  the  use  of  wide  brushes  is  the  lowering  of  the  tran- 
sition resistance  R^  due  to  the  greater  contact  area  of  the 
brush. 

There  are  a  number  of  purely  mechanical  conditions  upon 
which  depend  the  quality  of  commutation.  A  rough  commu- 
tator surface  causes  vibration  of  the  brushes,  which  results 
in  a  widely  varying  transition  resistance  and  subsequent 
sparking.  A  further  source  of  sparking,  more  especially  in 
high-speed  machines,  is  loose  commutator  bars.  Such  spark- 
ing produces  local  blackening  of  the  commutator  surface. 
An  open  or  discontinuous  armature  winding  and  a  reversed 
coil  occasion  severe  sparking.  The  limit  of  the  capacity  of 
a  machine  may  be  excessive  sparking  instead  of  excessive 
heating,  and  therefore  the  suppression  of  sparking  by  proper 
design  of  the  machine  is  of  utmost  importance. 


138  DYNAMO   ELECTRIC   MACHINERY. 


PROBLEMS. 

1.  Compute  the  field  ampere-turns  per  pole  necessary  to  com- 
pensate  for    demagnetization    in  a   i5~K.  W.,   i25~volt,    4-pole 
dynamo  having  a  wave-wound  armature,  the  winding  being  con- 
tained in  121  armature  slots,  with  4  conductors  per  slot.     The 
commutator  has   121  segments,  and  the  commutating  plane  is 
shifted  3  segments  ahead.     Dispersion  coefficient  =   1.25. 

2.  Calculate  the  field  ampere-turns  per  pole  necessary  to  neu- 
tralize the  distortional  effect  of  the  armature  current  of  a  6-pole 
generator  with  a  triplex  lap-wound  armature  having  a  total  of 
1086  conductors.     The  rated   current  of  the  machine  is  500 
amperes.     The  angle  of  brush  lead  is  8  degrees.     No-load  field 
ampere-turns  per  pole  =  7200. 

3.  For  the  dynamo  of  Prob.  4,  Chap.  IV,  calculate  the  addi- 
tional ampere-turns  required  to  compensate  for  armature  reac- 
tion.    The  armature  has  a  simplex  wave  winding  with  two  con- 
ductors per  slot ;  the  angle  of  brush  lead  is  5  degrees. 

4.  Determine  the  time  of  short-circuit  of  an  armature  coil 
undergoing  commutation  of  a  i2-pole,  350-K.  W.,  25o-volt,  70- 
rev.-per-min.  generator  having  a  commutator  56  inches  in  diam- 
eter with  448  segments.     The  brushes  exactly  cover  two  seg- 
ments with  the  intervening  insulation,  which  is  .03  inch  thick. 

5.  The  generator  armature  of  the  foregoing  problem  has  a 
simplex  lap  winding,  and  the  inductance  of  a  short-circuited 
coil  is  0.000030   henry.     Compute  the    reactance  voltage  per 
segment. 

6.  Determine  the  reactance  voltage  per  segment  of  a  1 2-pole 
300-K.  W.,   5oo-volt  generator  making  200  rev.  per  min.,  the 
constants  of  which  follow : 

Diameter  of  armature  =75  in. 

Gross  length  of  armature  =  18  in. 

Diameter  of  commutator  =50  in. 


PROBLEMS. 

Number  of  commutator  segments  =  489. 

Thickness  of  insulation  between  segments  =  0.04  in. 

Number  of  armature  slots,  as  per  Fig.  82  =489. 

Size  of  armature  conductors  =  J"x£"- 
Armature  is  lap  wound ;  two  conductors  per  slot. 

Brush  width  =  f  in. 


139 


-1 


Fig.  82. 


140  DYNAMO   ELECTRIC   MACHINERY. 


CHAPTER    VI. 

GENERATORS. 

Efficiemy  of  Operation. 

59.  Capacity  of  a  Dynamo.  —  The  capacity  of  a  genera- 
tor is  measured  by  the  power  it  can  develop,  that  is,  the 
capacity  varies  as  the  product  of  the  terminal  electromo- 
tive force  and  the  current  supplied  to  an  outside  circuit. 
The  E.M.F.  of  a  dynamo  depends  upon  the  speed,  the 
number  of  conductors,  and  the  number  of  magnetic  lines 
of  force  passing  through  the  armature,  §  39.  The  allow- 
able current  output  depends  primarily  upon  the  size  of  the 
armature  conductors,  so  that  these  may  carry  the  required 
current  without  excessive  heating.  The  larger  the  con- 
ductors, the  larger  must  be  the  armature  core,  other  things 
remaining  the  same.  Sometimes,  however,  commutation 
difficulties  limit  the  output  of  a  machine  rather  than  tem- 
perature elevation. 

The  E.M.F.  of  a  generator  may  be  raised  by  increasing 
the  speed,  the  number  of  conductors,  or  the  magnetic  flux 
through  the  armature.  The  speed  of  a  machine  is  limited 
by  considerations  of  mechanical  strength  and  economy  of 
material.  It  is  frequently  specified  in  that  the  generator 
is  to  be  directly  connected  to  a  steam  engine,  turbine,  or 
other  prime  mover,  or,  in  the  case  of  a  motor,  by  direct- 
coupling  to  the  machine  it  operates.  The  speed  of  small 
machines  is  greater  than  that  of  large  ones,  but  the  periph- 
eral velocity  for  nearly  all  sizes  lies  between  25  and  TOO 
feet  per  second  for  belt-driven  machines,  and  between  25 


GENERATORS. 


141 


and  50  feet  per  second  for  direct-connected  machines.  In 
turbo-generators  the  speeds  may  be  as  high  as  250  feet 
per  second.  The  speed  limits  of  modern  direct-current  gen- 
erators, in  revolutions  per  minute,  are  given  in  the  follow- 
ing table  : 


K.W. 

GENERATOR    SPEEDS. 

DIRECT-CONNECTED. 

BELT-DRIVEN. 

5 

4OO-80O 

650-2000 

10 

350-500 

6OO-I800 

20 
5° 

250-4OO 
180-350 

550-1600 
50O-I2OO 

100 

I  2O-3OO 

450-90O 

200 

100-250 

4OO-6OO 

500 

70-120 

3CO-400 

TOOO 

60-90 

1500 

55-85 

2OOO 

50-80 

The  number  of  inductors  on  a  given  armature  can  be 
increased  by  decreasing  the  size  of  the  wire.  Sufficient 
cross-section  must,  however,  be  provided  in  the  conductors 
to  enable  them  to  carry  the  maximum  current  of  the 
machine  without  causing  them  to  heat  to  such  an  extent 
as  to  endanger  the  insulation.  Good  practice  calls  for  from 
400  to  800  circular  mils  for  armature  conductor  cross-sec- 
tion per  ampere,  the  proper  value  depending  upon  the  heat- 
dissipating  facilities  in  the  machine.  The  smaller  values 
are  suitable  for  intermittently  operating  machines,  such  as 
elevator  or  crane  motors  ;  whereas  the  larger  values  are  for 
continuously  running  machines,  such  as  central-station 
generators. 

The  field  flux  of  a  generator,  for  a  path  of  constant  reluc- 
tance, depends  upon  the  magnetomotive  force  produced  by 
the  current  in  the  field  winding.  Increasing  the  number 


142 


DYNAMO   ELECTRIC   MACHINERY. 


of  turns  on  the  field  coils  or  raising  the  current  flowing  in 
them  would  increase  the  magnetic  flux  passing  through  the 
armature.  Decreasing  the  reluctance  of  the  path  of  the 
flux  yields  a  similar  result  ;  hence  the  desirability  of  small 
air-gaps,  magnetic  material  of  high  permeability,  and  short 
flux  paths  of  large  cross-section. 

Because  of  armature  reaction,  it  is  desirable  to  limit 
the  current  flowing  in  the  armature  conductors.  This  is 
accomplished  by  providing  numerous  paths  between  brushes 
for  the  armature  current,  so  that  only  a  small  portion  of 
the  total  current  flows  through  each  coil.  This  implies  the 
provision  of  a  suitable  number  of  field  poles,  which  is  there- 
fore a  necessary  condition  for  obtaining  satisfactory  oper- 
ation as  regards  sparking.  The  usual  limits  as  to  the 
number  of  poles  on  commercial  direct-current  generators 
are  given  below  : 


K.W. 

NUMBER  OF  POLES. 

*~1S 

2-4 

15-100 

4-6 

100-200 

6-8 

20O-300 

6-1  o 

300-500 

8-12 

50O-IOOO 

10-16 

IOOO-20OO 

12-24 

60.  Heating  of  Dynamos.  —  When  a  generator  delivers 
current,  there  is  a  continuous  production  of  heat  in  the 
armature  and  field  magnets,  which  represents  the  conver- 
sion of  some  electrical  energy  into  heat.  This  production 
of  heat  is  occasioned  by  eddy  current  and  hysteresis  losses 
in  the  iron,  copper  losses  in  both  armature  and  field  wind- 
ings, bearing  friction  and  windage,  pole-face  losses,  and 
commutator  losses.  The  temperature  of  the  machine, 


GENERATORS. 


143 


therefore,  continually  rises  until  a  temperature  is  reached 
at  which  as  much  heat  will  escape  per  unit  of  time  as  is 
generated  in  an  equal  period.  The  dissipation  of  heat 
takes  place  by  conduction,  air  convection,  and  radiation. 
The  ultimate  temperature  of  any  part  of  an  operating 
machine  depends  upon  the  emissivity  and  area  of  the  radi- 
ating surface  and  its  temperature  elevation  over  the  sur- 
rounding atmosphere.  Hence  it  is  necessary  to  design  each 
part  of  the  dynamo  so  that  its  temperature  rise  during 
continuous  full-load  operation  shall  not  exceed  a  certain 
prescribed  limit. 

The  ultimate  constant  temperature  is  usually  acquired 
after  from  6  to  18  hours  of  full-load  operation,  accord- 
ing to  the  size  and  construction  of  the  machine.  The 
temperature  of  the  armature  of  a  3OO-K.W.  generator 
operating  under  constant  full  load,  in  terms  of  time,  is 


10 


HOURS 

Fig.  83. 


shown  in  Fig.  83.  It  is  obviously  possible  to  obtain  a 
greater  output  for  a  short  time  without  excessive  rise  of 
temperature  ;  for  example,  a  machine  may  yield  25  per  cent 


144 


DYNAMO    ELECTRIC   MACHINERY. 


overload  capacity  for  two  hours  without  undue  temperature 
elevation.  Fig.  84  shows  the  ultimate  temperature  of  the 
armature  of  the  same  3OO-K.W.  generator  under  different 
loads,  a  constant  temperature  being  attained  at  each  load. 


150  . 300  450 

LOAD  IN   KILOWATTS 


Fig.  84. 

Two  methods  for  obtaining  the  rise  of  temperature  are 
in  common  use :  a,  by  a  thermometer ;  b,  by  increase  of 
electrical  resistance.  The  latter  method  is  to  be  preferred, 
and  should  be  used  wherever  practicable.  In  taking  the  tem- 
perature of  a  surface  of  the  machine  by  the  first  method, 
the  thermometer  bulb  should  be  laid  flat  against  that  sur- 
face and  be  covered  by  a  pad  of  cotton  sufficiently  small  to 
allow  normal  escape  of  heat  from  the  surface.  The  use  of 
the  cotton  pad  prevents  radiation  of  heat  from  the  bulb  of 
the  thermometer.  The  determination  of  the  temperature 
rise  of  a  winding  by  the  second  method  involves  the  meas- 
urement of  its  resistance  at  room  temperature  and  at  the 
ultimate  temperature  assumed  under  full-load  operation. 
Knowing  the  temperature  coefficient  of  resistance  of  the 
material  at  o°  C,  the  rise  in  temperature  may  be  com- 
puted, §  4. 

The  temperature  elevation  of  a  part   of   a  machine  as 


GENERATORS.  145 

determined  thermometrically  by  applying  a  thermometer  to 
the  hottest  accessible  portion,  may  frequently  be  less  than 
70  %  of  the  temperature  rise  as  computed  from  the  re- 
sistance measurements,  this  difference  depending  upon 
the  construction  of  the  part  under  test.  If,  however,  a 
thermometer  applied  to  a  winding  indicates  a  higher  tem- 
perature elevation  than  that  obtained  from  resistance 
measurements,  then  the  thermometer  indication  should  be 
accepted. 

At  rated  load  and  under  normal  conditions  of  ventilation, 
the  maximum  temperature  rise,  referred  to  a  standard  room 
temperature  of  25°  C,  should  not  exceed  50°  C.  for  field 
coils  and  armature,  as  measured  by  resistance  increase  ; 
55°  C.  for  commutator  and  brushes,  and  40°  C.  for  bearings 
and  other  parts  of  the  machine,  as  thermometrically  deter- 
mined. 

61.  Output  Coefficients. — The  capacity  or  rating  of  a 
machine  depends  to  a  great  extent  upon  its  heating.  In 
the  armature,  the  conductor  cross-section  must  be  such 
that  the  full-load  current  may  flow  through  the  winding 
without  producing  an  undue  temperature  rise.  The  em- 
ployment of  large  conductors  requires  large  armatures. 
Therefore  an  approximate  estimate  of  the  capacity  of  a 
machine  can  be  obtained  if  the  dimensions  and  speed  of 
the  armature  be  known. 

An  empirical  expression,  given  by  Kapp,  for  the  rated 
output  of  a  generator  in  kilowatts  is 

P  =  W\V, 

where       £  =  a  factor  called  the  output  coefficient, 
D  =  armature  diameter  in  inches, 
la  =  gross  axial  length  of  armature  core  in  inches, 
and         V  =  rev.  per  min.  of  the  armature. 


146 


DYNAMO   ELECTRIC   MACHINERY. 


The  value  of  the  output  coefficient  depends  upon  the  arma- 
ture diameter,  and  may  be  obtained  from  the  curve  of  Fig. 
85,  which  yields  results  typical  of  good  practice.  The  fore- 
going equation  is  useful 
for  rough  preliminary  de- 
sign purposes,  the  decision 
as  to  final  dimensions 
being  subject  to  consider- 
ations of  commutation 
and  voltage  regulation. 
_v62.  Losses  in  Armature 
Cores.  —  The  loss  of  en- 
ergy which  attends  the 
rotation  of  an  armature 
in  a  magnetic  field  is  due 
to  hysteresis  and  eddy 
currents  in  the  armature 
core.  In  order  to  reduce 
these  losses,  and  thereby 
to  prevent  an  excessive 
temperature  rise,  arma- 
ture cores  are  composed 
of  a  series  of  thin  disks 


100  150 

ARMATURE  DIAMETER  IN  INCHES 


Fig.  85. 


or   laminations,    insulated 
more    or  less  thoroughly 

from  each  other.  The  magnitudes  of  the  hysteresis  and 
eddy  current  losses,  as  shown  in  §§28  and  29,  depend 
upon  the  size  of  the  core,  the  magnetic  flux  density 
in  its  various  parts,  the  thickness  of  the  laminations, 
and  the  number  of  magnetic  cycles  passed  through  per 
second  by  the  iron.  Curves  of  hysteresis  and  eddy  cur- 
rent losses  in  armature  cores,  in  watts  per  cubic  inch 


GENERATORS. 


147 


and  per  pound,  expressed  in  terms  of  flux  density,  are  given 
respectively  in  Figs.  86  and  87,  for  both  25  and  60  cycles 


3456789       10 

KILO-MAXWELLS  PER  SQ.  CM, 

Fig.  86. 


11       12 


345        67        8         9 
KILO-MAXWELLS  PER  SQ.  CM 

Fig.  87. 


10       11 


per  second.     These  curves  are  plotted  from  values  calcu- 
lated by  means  of  the  formulae 


where  v  =  volume  of  iron  in  cubic  inches, 
f  =  cycles  per  second, 

/  =  thickness  of  laminations  in  mils  (usually  14), 
(Bm  =  max.  flux  density  in  maxwells  per  sq.  in., 
and        7)  —  hysteretic  constant  (taken  as  0.0021). 


148  DYNAMO   ELECTRIC    MACHINERY. 

63.  Armature  Copper  Loss. — When  a  generator  deliv- 
ers current  to  an  external  circuit,  this  current,  in  flowing 
through  the  armature  winding,  occasions  a  loss  of  energy, 
which  appears  as  heat.  The  magnitude  of  this  loss  at  any 
load  is  equal  to  the  product  of  the  armature  resistance  from 
the  positive  to  the  negative  brushes  (excluding  brush  tran- 
sition resistance)  and  the  square  of  the  total  current  of  the 
machine  at  the  definite  load.  If  5  be  the  number  of  con- 
ductors in  series  between  brushes,  that  is,  the  total  number 
of  conductors  on  the  armature  divided  by  q  ( =  number  of 
current  paths  through  armature  from  positive  to  negative 
brushes),  if  La  be  the  length  in  inches  of  the  embedded 
portion  of  one  conductor  plus  the  length  on  one  end  of  the 
exposed  or  free  portion,  and  if  Aa  be  the  cross-section  of 
an  armature  conductor  in  square  inches,  then  the  total 
resistance  of  the  armature  in  ohms  is 

p        o.S2$LaS 

Aa  =  — -          — —  » 

Aaq  I06 

where  the  constant  0.825  X  io~6  is  the  resistance  in  ohms 
between  opposite  faces  of  an  inch  cube  of  copper  at  75°  C. 
Values  of  q  for  various  types  of  armature  windings  are 
given  in  the  table  of  §  39. 

For  preliminary  design  purposes,  in  order  to  save  time, 
the  value  of  La  is  frequently  taken  as  the  sum  of  the 
embedded  length  of  a  conductor,  /a,  plus  1.5  times  the 
pole  pitch.  That  is,  the  quantity  1.5  Xp  is  an  estimation  of 
the  free  length  of  an  armature  conductor  on  one  end  of 
the  core.  Then 

La  =la+   1.5  Xp. 

The  armature  copper  loss  at  full  load  in  watts  is  there- 
fore  Pa  =  PRa, 


GENERATORS.  149 

where  /  is  the  full  load  current  of  the  dynamo  leaving  or 
entering  at  the  brushes. 

When  large  armature  conductors  pass  through  a  non- 
uniform  magnetic  field,  such  as  exists  under  the  field-pole 
corners,  eddy  currents  will  be  produced  in  them  because 
of  the  greater  E.M.F.  induced  in  one  side  of  the  conductor. 
This  loss  is  reduced  in  large  machines  by  using  several 
conductors  connected  in  parallel  instead  of  one  large  equiva- 
lent conductor. 

64.  Pole-Face  Losses.  —  In  dynamos  having  toothed 
armatures  the  reluctance  of  the  air-gap  between  the  arma- 
ture and  the  field  poles  is  less  opposite  a  tooth  than  oppo- 
site a  slot.  Consequently  more  lines  of  force  pass  through 
portions  of  the  pole  face  opposite  armature  teeth  than 
through  those  portions  opposite  the  slots  in  the  armature 
core.  As  the  armature  rotates  each  point  of  the  pole  face 
is  subjected  to  a  pulsating  flux,  thus  giving  rise  to  eddy- 
current  and  hysteresis  losses  in  the  pole  pieces.  To  min- 
imize this  effect  pole  pieces  are  often  constructed  of 
laminated  iron  or  sheet  steel. 

The'  magnetomotive  force  of  the  eddy  currents  tends  to 
equalize  the  flux  density,  and  therefore  the  pulsation  is 
confined  to  a  very  thin  surface  layer  of  the  pole  pieces. 
An  expression  for  the  pole-face  loss  of  a  dynamo  in  watts, 
given  by  Adams,  is 


where  (fc  is  the  average  pole-face  flux  density  in  maxwells 
per  square  inch  ;  Ap  is  the  total  pole-face  area  of  the 
dynamo  in  square  inches  ;  v  is  the  peripheral  velocity  of 
the  armature  in  feet  per  second  ;  jj.  is  the  permeability  of 
the  pole  face  ;  p  is  the  electrical  resistivity  of  the  pole  face 


ISO 


DYNAMO   ELECTRIC   MACHINERY. 


in  c,g.s.  units  (p  equals  about  1500);  k  is  1.6  times  the 
square  root  of  the  tooth  pitch  in  inches  for  solid  pole  pieces, 
and  4.1  times  the  thickness  of  the  laminations  in  inches 
divided  by  the  square  root  of  the  tooth  pitch  in  inches  for 
laminated  pole  shoes  ;  kf  is  a  constant  depending  upon  the 
ratio  of  slot  opening,  WQ,  to  the  radial  length  of  the  air- 
gap,  A,  and  may  be  taken  from  Fig.  88.  The  foregoing 
expression  has  received  experimental  verification. 


0.2 


8 

Wg, 

A 
Fig.  88. 


The  pole-face  loss  of  a  dynamo  is  usually  less  than  one 
per  cent  of  the  output  of  the  machine.  There  is  another 
source  of  energy  loss  in  the  pole  face  which  is  due  to  re- 
luctance pulsation  of  the  magnetic  circuit  of  the  machine 
occasioned  by  a  variation  in  the  number  of  teeth  under  a 
field  pole.  Large  air-gaps  and  chamfered  pole  corners 
practically  eliminate  the  flux  pulsation.  In  the  case  of 
laminated  pole  pieces,  there  are  some  additional  losses  due 


GENERATORS.  151 

to  transverse  bolts  and  screw  heads  which  serve  to  hold 
such  shoes  on  the  field  cores. 

65.  Excitation  Loss.  —  The  loss  of  power  due  to   the 
current  flowing  through  the  field-magnet  windings  for  pro- 
ducing the  magnetic  field  of  the  machine  is  easily  calcu- 
lated as  the  product  of  the  square  of  that  current  and  the 
resistance  of  the  winding.     Thus,  the  excitation  loss  of  a 
series-wound  dynamo  in  watts  is 

Pf=Iu'Ru,  (i) 

and  that  of  a  shunt-wound  machine  in  watts  is 

Pf  =  Isk2Rsk  =  ~->  (2) 

J^sh 

where  Ise  and  fsh  in  amperes  are  the  currents  flowing  in 
the  series  and  shunt  field  coils  respectively,  Rse  and  Rsh 
are  the  resistances  thereof  in  ohms  at  the  steady  running 
temperature  (§  49),  and  E  is  the  terminal  voltage  of  the 
generator.  In  a  compound-wound  machine  the  total  excita- 
tion loss  is  the  sum  of  the  foregoing  expressions,  or 

Pf  =  Ise*Rse+Ish*Rsh.  (3) 

Care  must  be  exercised  to  apply  this  equation  correctly  for 
long-shunt  and  short-shunt  compound-wound  dynamos. 

With  separately  excited  field  magnets  the  power  loss  in 
the  resistance  of  the  field-magnet  coils  alone  should  be  con- 
sidered, but  with  either  shunt-  or  series-wound  field  coils  the 
power  loss  in  the  accompanying  regulating  rheostat  should 
also  be  included,  since  this  apparatus  is  considered  an 
essential  part  of  the  machine. 

66.  Bearing  Friction  and  Windage.  —  As  an  armature 
revolves,  some  energy  is  wasted  in  bearing  friction  and 
windage,  and  this  loss  may  be  considered  independent  of. 


152 


DYNAMO   ELECTRIC   MACHINERY. 


the  load  on  the  machine.  Its  value  cannot  be  determined 
accurately,  but  may  be  estimated  by  means  of  the  curve 
in  Fig.  89,  given  by  Hobart.  The  loss  in  watts  due  to 


1200 

0 
800 

400 
1 

> 

X 

/ 

/ 

/ 

/ 

/ 

/ 

V  = 

1000 

/ 

[_ 

4000       80CO       12000 

Fig.  89. 

friction  and  windage,  Pfw,  is  plotted  against  the  product 
of  the  square  of  the  armature  diameter  in  inches  into  the 
axial  length  of  the  armature  over  the  end  connections  of 
the  winding.  The  latter  factor  may  be  considered  as  the 
gross  length  of  the  armature  plus  seven-tenths  of  the  pole 
pitch,  or  4  +  0.7  XP  inches.  The  curve  refers  to  a  speed 
of  1000  rev.  per  min.  ;  the  friction  and  windage  loss  at 
any  other  speed  is  taken  in  direct  proportion. 

This  loss  is  usually  less  than  one-half  per  cent  in  ma- 
chines of  over  1000  K.  W.  output,  and  may  be  2  to  3  per 
cent  in  machines  of  20  K.  W.  or  under. 

67.  Commutator  Loss.  —  The  transition  resistance  be- 
tween the  brushes  and  commutator  causes  a  drop  in  volt- 
age at  each  point  of  contact,  the  magnitude  of  which 


GENERATORS.  153 

depends  upon  the  quality  of  the  brush,  but  is  practically 
independent  of  commutator  speed,  brush  current  density, 
and  brush  pressure,  §  42.  This  drop  for  both  positive  and 
negative  brushes  varies  between  1.2  and  2.8  volts.  There- 
fore the  product  of  this  drop  times  the  current  leaving  or 
entering  at  the  brushes  gives  the  loss  due  to  the  brush 
transition  resistance. 

The  pressure  of  the  brushes  on  the  commutator  causes 
a  friction  loss.  This  quantity  may  be  expressed  as  equal  to 

746  nDcVfifF 

— - —  watts,  §  42 

33000  X  12 

where  Dc  —  commutator  diameter  in  inches, 

V  =  rev.  per.  min.  of  armature, 

/  =  coefficient  of  friction  (0.30  for  carbon  brushes 
and  0.25  for  copper  brushes), 

F  =  sum  of  pressures  of  all  brushes  on  commuta- 
tor in  pounds  ;  generally  1.25  Ibs.  per  sq.  in. 
of  rubbing  surface. 

To  allow  for  the  additional  loss  at  the  commutator  due  to 
sparking  at  the  brushes  and  currents  in  the  short-circuited 
segments,  which  cannot  be  determined  accurately,  six  per 
cent  is  usually  added  to  the  regular  commutator  loss. 
Therefore  the  total  commutator  loss  in  watts  may  be  ex- 
pressed as 

Pc  =  i.o6[(i.2  to  2.8)7  +  0.005 9 DcVfi'F]. 

68.  Temperature  Elevation.  —  The  temperature  eleva- 
tion of  any  part  of  a  dynamo  is  proportional  to  the  watts 
expended  in  that  part  and  inversely  proportional  to  its 
radiating  surface.  The  rise  of  temperature  will  be  influ- 
enced considerably  by  the  speed  of  the  armature  and  by 


154  DYNAMO   ELECTRIC   MACHINERY. 

the  effectiveness  of  the  ventilating  arrangements.  The 
temperature  rise  is  considered  separately  for  armature, 
field  coils,  and  for  commutator. 

The  total  losses  in  the  armature  comprise  the  eddy- 
current  and  hysteresis  losses  in  the  core  and  the  copper 
loss  in  the  winding.  The  true  radiating  surface  of  the 
armature  is  difficult  of  determination,  since  the  end  con- 
nections of  the  winding  and  the  surfaces  on  both  sides  of 
the  ventilating  ducts  assist  in  radiating  some  of  the  heat 
developed  in  the  armature.  It  is  more  convenient  to  con- 
sider a  surface  to  which  the  cooling  effect  may  be  regarded 
as  approximately  proportional,  and  such  is  the  external  cylin- 
drical surface  of  the  armature. 

For  well -ventilated  armatures  of  modern  dynamos,  the 
temperature  elevation  in  degrees  Centigrade  as  thermo- 
metrically  measured  may  be  obtained  from  the  following 
expression  due  to  Arnold  : 

b  (P    _L_    PL   -|_    p  \ 

rKi\J-  e  ~   •*-  h     i     J-  a) 
n.   — 


tt  +  0.7  Ap)  (i  +  .031;) 

where  Pe  +  Ph  =  core  losses  in  watts,  §  62, 

Pa  =  armature  copper  loss  in  watts,  §  63, 
D  =  armature  diameter  in  inches, 
la  +  0.7  Ap  =  axial  length  of  armature  over  end  con- 
nections in  inches,  §  66, 
v  =  peripheral   velocity  of    armature  in  feet 

per  second, 

and  k^  =  a  constant  the  value  of  which  may  be 

taken  as  55. 

The  temperature  rise  of  the  field  coils  depends  upon  the 
depth  of  the  winding,  the  heat  emissivity  of  the  bobbins 


GENERATORS.  155 

upon  which  the  wire  is  wound,  and  the  effect  of  the 
fanning  action  of  the  revolving  armature.  For  multipolar 
machines  of  modern  design,  the  temperature  rise  in  degrees 
Centigrade,  as  obtained  from  resistance  measurements,  may 
be  calculated  from  the  following  expression  also  given  by 
Arnold, 


where  P/  is  the  total  excitation  loss  of  the  dynamo  in 
watts,  §  65,  A/  is  the  area  of  the  exposed  surface  of  all 
the  field  coils  in  square  inches,  and  k^  is  a  constant  the 
value  of  which  may  be  taken  as  90. 

For   the    temperature    rise    of    commutators,   the   same 
authority  gives  the  following  empirical  equation  : 

kfc 


n  Delc  (i  +  .031;) 

where  Tc  =  temperature   elevation  of    the  commutator  in 

degrees  Centigrade, 
•  Pc  =  commutator  loss  in  watts,  §  67, 
Dc  =  commutator  diameter  in  inches, 
lc  =  length  of  commutator  in  inches, 
v  =  peripheral  velocity  of  commutator  in  feet  per 

sec., 

and       £3  =  a  constant  depending  upon  degree  of  ventila- 
tion ;  its  value  may  be  taken  as  20. 

69.  Efficiency.  —  The  efficiency  of  a  machine  is  defined 
as  the  ratio  of  its  net  power  output  to  its  gross  power 
input.  It  may  also  be  defined  as  the  ratio  of  the  net 
power  output  to  the  sum  of  the  net  power  output  and  the 
total  losses.  If  P0  be  the  output  in  watts,  and  Pin  be  the 


156  DYNAMO   ELECTRIC   MACHINERY, 

input  to  a  dynamo  in  watts,  then  the  efficiency  is 

Po  Po 


Pin         Po  +   (Ph  +  Pe  +  Pa  +  Pp  +  Pf  +  Pfw  +  Pc) 

the  various  losses  being  determined  as  in  §§  62  to  67. 
The  efficiency  of  a  machine  at  full  load  should  be  deter- 
mined at  the  ultimate  temperature  assumed  under  con- 
tinuous operation  at  rated  load,  referred  to  the  standard 
engine-room  temperature  of  25°  C. 

The  electrical  power  delivered  by,  or  supplied  to,  a 
dynamo  should  be  measured  at  the  terminals  of  the 
machine,  and  is  given  by  the  product  of  the  terminal  volt- 
age and  the  ampere  output.  The  mechanical  power  should 
be  measured  at  the  pulley,  gearing,  coupling,  etc.,  thus 
excluding  the  losses  in  these  devices,  but  including  the 
bearing  friction  and  windage.  If,  however,  a  generator  be 
mounted  directly  upon  the  shaft  of  a  prime  mover  so  that 
it  cannot  be  separated  therefrom,  the  frictional  losses  in  the 
bearings  and  in  windage  may  be  disregarded  in  determining 
the  efficiency  of  the  dynamo,  owing  to  the  difficulty  in  appor- 
tioning these  losses  between  prime  mover  and  generator. 

Where  a  machine  has  auxiliary  apparatus,  such  as  an 
exciter,  the  power  lost  in  the  auxiliary  apparatus  should  not 
be  charged  to  the  machine,  but  to  the  plant  consisting  of 
machine  and  auxiliary  apparatus  taken  together.  In  such 
cases  plant  efficiency  should  be  distinguished  from  machine 
efficiency. 

The  efficiency  of  a  dynamo  increases  with  the  size,  being 
low  on  small  machines,  and  quite  high  on  the  larger  ones. 
The  efficiencies  to  be  expected  of  modern  direct-current 
compound-  or  shunt-wound  generators  of  various  sizes  at 
full  load  are  shown  by  the  curve  of  Fig.  90. 


GENERATORS. 


157 


100, 


70 


PERCENTAGE   OUTPUT 

Fig.  91. 

The  efficiency  of  a  compound-  or  shunt -wound  dynamo 
is  small  at  low  outputs  because  the  practically  constant 
core  losses,  friction  and  windage  loss,  and  shunt-field  exci- 
tation loss  are  then  large  in  comparison  with  the  power 
output.  Fig.  91  shows  how  the  efficiency  of  a  certain 


158  DYNAMO   ELECTRIC    MACHINERY. 

2OO-K.W.  compound-wound  generator  increases  with  the 
output.  Curves  are  also  given  in  this  figure  which  show 
the  variation  of  the  different  losses  with  the  output  of  the 
generator.  Since  the  distribution  of  the  magnetic  and 
electrical  losses  of  a  generator  lies  within  the  discretion  of 
the  designer,  it  is  possible  to  so  design  a  machine  as  to  have 
its  point  of  maximum  efficiency  at  full  load  or  at  some  other 
specified  load.  As  a  rule,  however,  the  exact  location  of 
the  maximum  efficiency  is  hardly  considered  in  designing 
a  dynamo,  since  the  efficiency  near  the  maximum  value  is 
fairly  constant  over  wide  variations  of  load. 

70.  Coefficient  of  Conversion.  —  The  coefficient  of  con- 
version of  a  generator  is  the  ratio  of  the  total  electrical 
energy  developed  in  the  armature  winding  to  the  total  me- 
chanical energy  supplied  to  the  armature.  This  is  some- 
times called  the  efficiency  of  conversion,  but  to  distinguish 
it  from  efficiency  as  denned  in  the  foregoing  article,  it  is 
better  to  use  the  term  coefficient  of  conversion.  This  co- 
efficient is  always  less  than  unity,  and  is  expressed  by 

Einl 


Einl  +  Pe  +  Ph  +  PfW  +  Pp 

where  Ein  is  the  actual  voltage  generated  in  the  armature, 
i.e.,  internal  E.M.F.,  I  is  the  current  of  the  generator  in 
amperes  leaving  or  entering  at  the  brushes,  and  Pe,  Ph, 
Pfw,  and  Pp  are  respectively  the  eddy-current,  hysteresis, 
friction  and  windage,  and  pole-face  losses  of  the  machine 
in  watts. 

71.  Economic  Coefficient.  —  Some  of  the  electrical  power 
developed  in  a  generator  armature  is  consumed  in  overcom- 
ing the  resistance  of  the  armature  winding,  some  is  wasted 
at  the  commutator,  and  some  is  expended  in  exciting  the 


GENERATORS.  159 

field  magnets  ;  the  remainder  being  delivered  as  useful 
power  to  the  external  circuit,  or  load.  The  ratio  of  this 
useful  electrical  energy  to  the  total  electrical  energy  devel- 
oped in  the  armature  is  known  as  the  economic  coefficient, 
and  sometimes  as  the  electrical  efficiency.  Hence,  the  eco- 
nomic coefficient  may  be  expressed  as 

EjJ  -   (Pa  +  Pc  +  Pf) 


where  Pa,  Pc,  and  Pf  are  respectively  the  armature  copper 
loss,  commutator  loss,  and  excitation  loss  of  the  generator 
in  watts. 

The  efficiency,  or,  as  it  is  sometimes  called,  commercial 
efficiency,  of  a  generator  is  evidently  the  product  of  the  con- 
version and  economic  coefficients,  or 


72.  Magnetos.  —  Magnetos  or  magneto-generators  are 
dynamos  in  which  the  magnetic  flux  is  set  up  by  perma- 
nent '  magnets.  Since  the  flux  density  in  this  type  of 
machine  is  necessarily  low,  for  a  given  flux  .more  iron  must 
be  used  than  in  machines  having  their  fields  produced  by 
electro-magnets.  Therefore  the  application  of  magnetos 
is  limited  to  purposes  requiring  a  relatively  small  amount 
of  energy,  such  as  telephone  signaling,  automobile  ignition 
work,  and  testing  of  electrical  circuits. 

Magnetos  are  generally  alternating-current  machines  and 
provided  with  slip  rings  or  contact  studs  instead  of  com- 
mutators. The  armatures  are  usually  of  the  Siemens 
type,  wound  with  many  turns  of  fine  wire,  and  mounted 
so  that  they  may  be  rapidly  rotated  between  the  poles  of 


160  DYNAMO   ELECTRIC   MACHINERY. 

permanent  horseshoe  magnets.  Fig.  92  shows  a  telephone 
magneto-generator  manufactured  by  the  Western  Electric 
Company.  To  the  armature  shaft  is  affixed  a  pinion  which 
meshes  with  a  gear  wheel  turned  by  hand.  Such  machines 
are  designed  to  ring  a  call-bell  or  telephone  ringer  through 
an  external  resistance  as  high  as  50,000  ohms.  The 
armature  windings  of  magnetos  have  resistances  between 
300  and  600  ohms  depending  on  the  type  of  service  for 
which  they  are  designed.  These  generators  are  provided 


Fig.  92. 

with  "shunts,"  which  afford  a  by-path  of  low  resistance 
around  the  armature  when  not  in  use,  or  devices  which 
normally  hold  the  armature  circuits  open  and  close  them 
when  the  generators  are  operated.  The  air  gaps  of  such 
machines  may  be  as  low  as  o.oi  inch  without  introducing 
operative  difficulties. 

73.  Constant-Potential  and  Constant-Current  Supply. — - 
There  are  in  use  two  systems  of  electrical  distribution : 
(a)  at  constant  potential,  (&)  with  constant  current.  In 
the  former  system,  lamps,  motors,  or  other  types  of  elec- 


GENERATORS.  l6l 

trical  apparatus  are  connected  in  parallel  with  each  other 
across  the  supply  mains.  To  secure  satisfactory  operation, 
it  is  necessary  to  maintain  a  constant  voltage  between 
these  mains,  so  that  if  some  of  the  load  be  disconnected, 
or  more  load  be  added,  the  current  flowing  in  the  remain- 
ing lamps,  motors,  etc.,  will  stay  unchanged.  This  method 
of  supplying,  at  any  point  of  usage,  current  at  a  constant 
potential  irrespective  of  the  load  which  is  there  or  at  other 
points  of  the  system,  is  very  generally  used  in  the  distri- 
bution of  electrical  energy  for  incandescent  electric  light- 
ing, for  operation  of  constant-pressure  motors,  and  for 
electric  traction.  The  great  sensitiveness  of  the  light  in- 
tensity of  incandescent  lamps  to  a  change  in  voltage,  the 
candle  power  varying  perhaps  as  the  fourth  power  of  the 
voltage,  requires  that  the  voltage  across  electric-lighting 
supply  mains  vary  less  than  3  per  cent  of  its  rated  value. 
In  electric  traction,  where  the  load  is  exceedingly  variable, 
particularly  in  trunk-line  operation,  constant-potential  dis- 
tribution can  only  be  approximated.  Frequently  a  drop 
of  25  per  cent  is  allowed. 

For  lighting  by  arc  lights  where  considerable  energy  is 
expended  at  the  points  of  illumination,  and  where  these 
points  are  separated  from  each  other  by  considerable  dis- 
tances, it  is  sometimes  economical  and  desirable  to  connect 
the  lamps  in  series.  For  satisfactory  operation  the  current 
in  the  circuit  should  be  maintained  constant,  so  that  if 
more  lamps  be  put  in  service,  or  some  taken  out,  the  volt- 
age across  the  remaining  lamps  will  be  unchanged.  A 
lamp  connected  to  such  a  system  may  be  cut  out  by  short- 
circuiting  it. 

The  advantage  of  constant-current  distribution  for  town 
lighting  is  the  economy  of  copper  for  the  supply  mains. 


162  DYNAMO   ELECTRIC    MACHINERY. 

The  line  can  be  made  of  much  smaller  wire  than  in  the 
case  of  a  constant-pressure  circuit,  for  on  a  constant-current 
circuit  as  the  load  increases  the  power  or  energy  trans- 
mitted is  increased  by  raising  the  potential,  the  current 
remaining  unaltered ;  while  in  a  constant-pressure  circuit 
an  increase  of  load  is  met  by  an  increase  of  current,  and 
the  supply  mains  must  be  of  sufficient  size'  to  safely  carry 
the  required  maximum  current.  The  size  of  wire  neces- 
sary is  dictated,  not  by  the  energy  transmitted,  but  by  the 
current  flowing,  hence  a  wire  large  enough  to  supply  just 
one  lamp  of  a  constant-pressure  circuit  can  supply  all  the 
lamps  of  a  constant-current  circuit. 

Constant-Potential  Generators. 

74.   Characteristic  Curves  of  Shunt-Wound  Generators. 

—  The  operation  of  any  dynamo,  can  best  be  studied  by 
inspection  of  a  curve  which  shows  the  relation  existing  be- 
tween the  current  generated  or  supplied  by  the  machine 
and  the  voltage  under  which  it  operates.  Such  curves  are 
called  characteristic  curves,  and  they  are  generally  plotted 
with  current  strengths  as  abscissae  and  voltages  as  ordi- 
nates.  The  characteristic  curve  of  a  shunt-wound  gener- 
ator is  shown  as  E  in  Fig.  93,  and  it  is  seen  therefrom  that 
the  terminal  voltage  decreases  slightly  as  the  current  out- 
put of  the  machine  increases,  the  speed  of  the  generator 
being  maintained  constant. 

To  obtain  the  characteristic  curve  of  a  shunt-wound  gen- 
erator experimentally  the  machine  is  run  at  normal  speed, 
and  readings  are  taken  of  terminal  voltage  and  current  out- 
put, the  setting  of  the  field  rheostat  being  fixed  during  the 
test.  The  setting  of  this  rheostat  may  be  that  giving  rated 


GENERATORS. 


163 


voltage  either  at  no  load  or  at  full  load.  Fig.  93  indicates 
the  latter  condition.  In  some  small  machines  the  voltage 
can  be  reduced  to  zero  without  causing  excessive  sparking 
or  extreme  temperature  elevation,  but  as  a  rule  the  com- 
plete characteristic  is  obtained  only  when  the  field  rheostat 
is  adjusted  for  a  voltage  much  below  the  rated  voltage  of 
the  machine. 


40 


50  75  100 

PERCENT    FULL- LOAD    CURRENT 


Fig.  93- 

The  characteristic  curve  of  a  strictly  constant-potential 
generator  would  be  a  straight  horizontal  line,  since  this 
indicates  that  the  voltage  remains  the  same  at  all  loads. 
The  terminal  voltage  of  a  shunt-wound  dynamo  at  constant 
field  excitation  and  speed  decreases  slightly  as  the  load  in- 
creases, because  of  the  armature  resistance  drop  and  arma- 
ture reaction,  §  50.  The  armature  resistance  drop,  being 
the  product  of  the  armature  current  and  resistance,  is  prac- 
tically a  linear  function  of  the  load  (the  change  in  resist- 
ance due  to  heating  occasioned  by  increased  current  may 
be  neglected),  and  may  be  plotted  as  a  straight  line,  as  in 


164  DYNAMO   ELECTRIC   MACHINERY. 


ig-  93-  The  total  voltage  generated  is  obtained  by  add- 
ing the  armature  resistance  drop  to  the  terminal  voltage. 
Thus,  the  curve  of  total  voltage,  Et,  is  plotted  by  adding 
the  ordinates  of  E  and  those  of  the  resistance-drop  curve. 
The  difference  between  Et  and  the  no-load  terminal  voltage 
of  the  machine  shows  the  effect  of  armature  reaction. 

The  drop  in  terminal  voltage  is  at  first  due  chiefly  to 
the  drop  resulting  from  armature  resistance.  As  the  cur- 
rent increases,  the  effects  of  armature  reaction  and  satura- 
tion of  the  magnetic  circuit  become  evident.  This  soon 
becomes  the  predominating  cause  of  voltage  drop,  and  to 
such  an  extent  that  the  curve  turns  back  toward  the 
origin.  When  the  resistance  in  the  external  circuit  is  zero, 
of  course  no  current  flows  through  the  field,  and  the  few 
volts  then  produced  are  due  to  residual  magnetism.  Unless 
the  field  excitation  is  kept  constant  in  determining  the 
terminal  voltage  curve  of  a  generator,  it  should  be  re- 
membered that  the  difference  between  Et  and  the  no-load 
terminal  voltage  is  also  due  to  a  decrease  of  the  field  cur- 
rent occasioned  by  the  fall  of  potential  at  the  terminals 
of  the  field  winding. 

The  voltage  of  a  shunt  machine  generally  increases 
more  rapidly  than  the  speed.  An  increase  of  speed  not 
only  increases  primarily  the  number  of  volts  generated,  but 
also  increases  the  armature  flux  because  of  increased 
excitation.  The  condition  of  the  magnetic  circuit  as  re- 
gards saturation  determines  whether  this  secondary  influ- 
ence shall  be  great  or  small. 

75..  Voltage  Regulation.  —  Shunt-wound  generators  are 
so  designed  that  the  lowering  of  terminal  voltage  from  no 
load  to  full  load  shall  be  as  small  as  is  consistent  with 
economy  and  practicability.  Such  machines  are  particu- 


GENERATORS.  165 

larly  adapted  for  constant-potential  distribution.  The 
maintenance  of  a  perfectly  constant  terminal  voltage  is 
effected  by  the  use  of  regulating  field  rheostats. 

Suppose  the  field  rheostat  of  a  shunt-wound  generator 
to  be  adjusted  for  obtaining  the  rated  voltage  of  the 
machine  at  full  load.  Upon  disconnecting  the  load  and 
leaving  the  rheostat  setting  unaltered,  the  terminal  voltage 
of  the  generator  increases.  This  change  of  voltage  from 
full  load  to  no  load  at  constant  speed  when  expressed  as 
a  percentage  of  the  rated  full-load  voltage,  is  termed  the 
voltage  regulation  of  the  generator.  Thus  the  regulation 
of  the  generator  of  the  foregoing  section  at  full  load,  as 
obtained  from  Fig.  93,  is 

128  —  1 10  ^ 
=  0.163  or  16.3  per  cent. 

The  regulation  of  a  separately  excited  generator  should 
be  determined  at  constant  excitation.  The  regulation  of 
a  generator  unit,  consisting  of  a  generator  united  with  a 
prime  mover,  should  be  determined  at  constant  conditions 
of  the  prime  mover ;  i.e.,  constant  steam  pressure,  head, 
etc.  It  would  include  the  inherent  speed  variations  of  the 
prime  mover.  For  this  reason  the  regulation  of  a  genera- 
tor unit  is  to  be  distinguished  from  the  regulation  of  either 
the  prime  mover  or  of  the  generator  contained  in  it,  when 
taken  separately. 

76.  Hand  Regulation.  —  To  maintain  a  perfectly  con- 
stant terminal  voltage  at  increased  load  necessitates  an 
increase  in  the  total  E.M.F.  generated  in  the  machine. 
An  inspection  of  the  formula  for  the  electromotive  force 
of  a  generator, 


1 66 


DYNAMO   ELECTRIC   MACHINERY. 


shows  that  the  only  quantity  that  it  is  practical  to  vary  is 
the  magnetic  flux  through  the  armature  4>m.  This  can 
easily  be  accomplished  by  regulating  the  amount  of  re- 
sistance in  a  rheostat,  which  is  in  series  with  the  field 

coils,  and  which  therefore 
governs  the  amount  of  cur- 
rent in  them,  as  in  Fig.  94. 

In  distributing  current  for 
use  among  a  number  of  con- 
sumers the  current  is  carried 
to  feeding-points  which  are 
near  the  locality  they  supply, 
but  may  be  distant  from  the 
station.  It  is  desirable  to 
keep  the  pressure  at  these 
points  at  a  constant  value,  irrespective  of  the  varying  loss  of 
potential  that  is  going  on  because  of  the  resistance  of  the 
conductors  leading  to  them.  To  achieve  this  end  feeders 
are  employed  to  carry  the  current  to  the  feeding-points. 
Each  feeder  is  accompanied  by  a  pilot  wire  imbedded  in 
the  insulation.  At  the  feeding-point  the  pilot  wires  are 
attached  to  the  feeder  terminals,  and  at  the  station  end  are 
attached  to  a  voltmeter,  so  that  the  station  attendant  can 
regulate  the  pressure  not  at  the  machine  terminals  but  at 
the  distant  distributing  point. 

77.  Field  Rheostats.  —  For  varying  the  current  in  the 
shunt  field  coils  of  generators,  it  is  usual  to  employ  field 
rheostats  which  may  be  mounted  on  the  station  switch- 
board together  with  the  usual  indicating  instruments,  or 
on  a  separate  frame.  Such  rheostats  consist  essentially 
of  high-resistance  wire  or  ribbon  with  numerous  taps  con- 
nected to  a  series  of  contact  studs  over  which  moves  a 


GENERATORS. 


I67 


contact  arm.  The  resistance  units  may  be  in  the  form 
of  cards,  bars,  bobbins,  or  grids,  according  to  the  capacity 
required. 

A  field  rheostat,  manufactured  by  the  General  Electric 
Company,   is   shown  in  Fig.    95.      It  is   arranged  to  be 


Fig.  95. 


placed  on  the  back  of  switchboards  with  the  regulating 
handle  projecting  in  front.  *  The  resistance  units  are  of  the 
card  form,  and  are  constructed  by  winding  the  resistance 
ribbon  on  tubes  of  asbestos  which  are  subsequently  pressed 
flat.  These  cards  are  then  assembled,  with  interposed 
asbestos,  in  sufficient  numbers  to  make  up  the  required 


1 68 


DYNAMO   ELECTRIC   MACHINERY. 


resistance  of  the  rheostat.  Iron  plates,  somewhat  wider 
than  the  cards,  are  introduced  at  intervals,  and  thus 
increase  the  radiating  surface.  Numerous  taps  are 
taken  from  the  resistance  units  to  the  various  contact 
studs. 

Fig.  96  illustrates  a  rear  view  of  a  Westinghouse  rheo- 
stat with  the  bottom  plate  removed.     The  resistance  unit 


Fig.  96. 


is  of  the  bar  type,  so  called  because  the  resistance  wire 
is  wound  on  flat  iron  bars,  but  insulated  therefrom  by  a 
layer  of  fireproof  insulating  material.  By  varying  the 
size  of  the  wire,  resistances  may  be  wound  of  from  .03 
to  400  ohms  per  linear  inch  of  the  bar,  with  a  maxi- 
mum capacity  of  4  watts  per  square  inch  of  surface  on 
one  side. 

Field  rheostats  for  very  large  generators  consist  of  re- 
sistance units  in  the  form  of  iron  grids  supported  in 
frames,  which  are  mounted  directly  on  the  floor  at  some 


GENERATORS. 


169 


convenient  point  near  the  switchboard.  Fig.  97  shows 
such  a  rheostat  made  by  the  Westinghouse  Electric  and 
Manufacturing  Company. 


Fig.  97- 


Another  form  of  field  rheostat,  made  by  the  Cutler- 
Hammer  Manufacturing  Company,  is  shown  in  Fig.  98. 
In  this  rheostat  the  heat  generated  is  not  radiated  directly 
from  the  surface  of  the  wire,  but  is  conducted  to  a  sup- 
porting plate,  which  then  becomes  the  radiating  surface. 
The  resistance  wires,  contacts  and  lever  are  mounted  on  a 
base  of  insulating  material,  the  whole  being  carried  by  an 
iron  casing,  which  prevents  the  possibility  of  contact  with 


I/O  DYNAMO   ELECTRIC   MACHINERY. 

the  heat  radiating  portion  of  the  rheostat.  Owing  to  the 
increased  radiating  surface  thus  obtained,  a  shorter  and 
smaller  wire  can  be  used  for  a  given  volt-ampere  capacity 
than  if  the  wire  were  merely  exposed  to  the  air.  No  con- 
sideration of  the  mechanical  strength  of  the  wire  enters 
into  the  design  of  this  resistance,  since  it  is  supported  and 
protected  by  an  insulating  compound. 


Fig.  98. 

When  large  generators,  such  as  are  used  in  railroad 
work,  have  their  field-circuits  opened,  the  E.M.F.  self- 
induced  by  the  disappearance  of  the  flux  in  the  fields  is 
liable  to  reach  such  a  magnitude  as  to  pierce  the  insulation 
of  the  field  coils  and  destroy  their  usefulness.  To  obviate 
this,  before  the  field  circuit  is  broken,  the  field  coils  are 
connected  (Fig.  99)  through  a  high  discharge  resistance, 
and  the  current  in  them  is  allowed  to  decay  slowly.  It  is 


GENERATORS. 


thus  unattended  with  any  destructive  potential  differences. 
Arc  lights  have  in  several  instances  been  used  for  this  pur- 
pose instead  of  high  resistances. 


1 

Binding  Posts 

Field  Switch 

°4 

07 

o  : 

09; 
O    - 

On  < 

&\ 
3 

O 

<0oo'^  "ob03\° 

tf   ©  (Sg- 

V                                         35  O 

°°?9oo]jto00«  o 

r 

6J 

< 
< 

o= 

^ 

» 

> 

>      '._ 
»  ," 
fc" 
» 
»"« 
• 
I 

Tleld 

Parallel  Resistance*                           Resistan 
Rheostat  Switch 

Pilot  Lamp 

• 

7MKMT 

u\ 

V^" 

Armature 


Fig.  99. 


78.  Self-Regulation.  —  By  far  the  most  elegant  method 
of  constant  potential  regulation  is  that  in  which  the  main 
current  of  the  machine  is  utilized  in  maintaining  constant 
the  magnetic  flux  through  the  armature.  This  is  accom- 
plished by  passing  all  or  the  greater  part  of  the  current 
flowing  in  the  armature  a  few  times  around  the  field 
magnets,  so  that  an  increased  load  on  the  armature  in- 
creases the  magnetizing  ampere-turns  of  the  field  coils. 
These  series  turns,  when  rightly  proportioned,  can  be  made 


1/2  DYNAMO   ELECTRIC   MACHINERY. 

to  compensate  for  a  part,  for  all,  or  for  even  more  than  all 
of  the  drop.  This  device  can  be  used  in  connection  with 
any  other  form  of  excitation,  as  permanent  magnets,  sep- 
arate excitation,  or  shunt  excitation.  In  the  last  case,  the 
dynamo  is  said  to  be  compound  wound,  as  described  in 
§46.  If  the  machine  is  designed  to  maintain  a  constant 
pressure  at  some  distant  feeding-point,  instead  of  at  the 
machine  terminals,  the  machine  is  said  to  be  over-com- 
pounded, since  the  potential  at  the  terminals  will  rise  on 
increase  of  load.  From  3  to  5  per  cent  over-compound- 
ing is  frequent  in  machines  used  to  supply  lighting  circuits, 
and  10  per  cent  over-compounding  is  usual  in  railway 
generators. 

79.  Characteristic  Curves  of  Compound- wound  Gen- 
erators. —  As  a  compound-wound  machine  is  essentially 
a  shunt-wound  generator  provided  with  a  series  field  wind- 
ing, the  characteristic  curve  thereof  would  be  the  resultant 
of  the  shunt  characteristic  and  the  series  characteristic. 
The  form  of  these  curves  is  shown  in  Fig.  100,  the  shunt 
characteristic  being  the  same  as  that  for  shunt-wound 
dynamos.  The  voltage  induced  in  the  armature  by  the 
increase  of  magnetic  flux  due  to  the  current  in  the  series 
turns  is  proportional  to  the  current.  As  the  load  in- 
creases this  voltage  will  increase,  and,  if  the  series  wind- 
ing be  properly  proportioned,  the  increase  of  the  voltage 
due  to  the  current  in  the  series  turns  may  neutralize  the 
decrease  of  the  main  voltage  occasioned  by  armature 
resistance  and  reaction.  The  form  of  the  curves  of  Fig. 
100  indicates  that  such  neutralization  can  occur  at  only 
one  load.  If  the  compensation  be  complete  at  full  load, 
the  machine  is  said  to  be  flat-compounded.  The  compound 
characteristic  for  flat-compounding  is  shown  by  the  broken 


GENERATORS. 


173 


line,  and  that  for  over-compounding  is  shown  by  the  full 
line  in  the  figure. 


60  75  ICO 

PERCENT  FULL- LOAD   CURRENT 


Fig.  100. 

The  degree  of  compounding  may  be  changed  by  varying 
either  the  current  flowing  through  the  series  winding  or 
the  number  of  turns  on  it.  In  practice  it  is  usual  to  pro- 
vide more  series  turns  than  required,  and  to  place  an  ad- 
justable resistance  across  the  terminals  of  the  series  field 
coils.  The  full  armature  current  therefore  divides  between 
this  resistance  and  the  series  coils,  and  the  amount  flowing 
through  the  latter  may  be  adjusted  for  the  required  com- 
pounding. 

In  over-compounded  machines,  the  voltage  regulation  is 
the  ratio  of  the  maximum  difference  in  voltage  from  a 
straight  line  connecting  the  no-load  and  full-load  values  of 
terminal  voltage  as  function  of  the  current,  to  the  full-load 
terminal  voltage. 

80.  Railway  and  Lighting  Generators.  —  The  tendency 
of  modern  engineering  practice  is  to  install  lighting  gener- 


174 


DYNAMO   ELECTRIC    MACHINERY. 


ators  which  are  directly  connected  to  the  prime  mover. 
Owing  tp  the  inherent  speed  of  steam  engines  being  smaller 
than  that  of  generators,  direct-connected  armatures  are 
designed  to  run  at  a  lower  speed  than  belt-driven  ones. 
Economical  construction  demands  that  they  be  of  the  mul- 
tipolar  type.  They  require  less  floor  space  per  kilowatt  than 
the  belt-driven  machines  ;  and  this  is  a  question  of  consid- 
erable importance  in  many  installations.  They  have  a 
higher  efficiency  of  operation  consequent  upon  the  elimina- 
tion of  losses  in  belting  and  countershafting.  They  also 
permit  of  operation  of  isolated  plants  in  residences  and 
other  places  where  the  noise  resulting  from  belt-driven 
machinery  would  not  be  tolerated. 

In  order  that  standard  generators  may  be  easily  con- 
nected with  engines  of  any  make,  and  vice  versa,  commit- 
tees from  engineering  societies  have  recommended  the 
adoption  of  the  following  standard  sizes,  speeds,  and  ar- 
mature shaft  fits :  — 


Sizes  in  K.W.  Capacity  . 

5 

7-5 

10 

i.S 

20 

2.S 

.So 

Speeds  in  Rev.  per  Min.  . 

45° 

42  s 

400 

37  S 

3  So 

32  s 

290 

Armature  Fit  in  Inches   . 

3 

3 

3^ 

3% 

4 

4 

5 

Sizes  in  K.,  W.  Capacity  . 

7S 

100 

125 

'.So 

200 

250 

3oo 

Speeds  in  Rev.  per  Min.  . 

27.S 

250 

2.35 

220 

200 

190 

1  80 

Armature  P'it  in  Inches    . 

6 

7 

7^ 

8 

9 

10 

n 

Fig.  1 01  shows  a  i6oo-K.  W.,  i6-pole,  100  rev.  per 
min.  General  Electric  Company  direct-connected  engine- 
driven  railway  generator.  These  generators  are  built  in 
sizes  from  100  K.  W.  to  2700  K.  W.,  and  are  designed  to 
yield  the  prevailing  full-load  railway  voltages  of  550,  575, 
or  600  volts.  The  field-magnet  yoke  is  of  cast  iron,  circu- 


GENERATORS. 


175 


lar  in  shape  and  of  oval  or  rectangular  cross-section.  The 
frame  is  divided,  the  upper  half  being  fastened  to  the  lower 
by  concealed  bolts.  The  poles  are  solid  steel  castings 


Fig.  101. 


bolted  to  the  frame,  and  may  be  removed  laterally  without 
taking  out  the  armature.  Commutating  poles  are  provided 
in  most  sizes,  to  compensate  for  armature  reaction,  thus  in- 


176  DYNAMO  ELECTRIC   MACHINERY. 

suring  good  commutation  at  all  loads.  The  armature  spider 
is  equipped  with  vanes  which  fan  air  through  the  ventilat- 
ing passages  formed  through  the  laminations  and  wind- 
ings and  around  the  poles,  thus  improving  ventilation.  The 
brush-holder  mechanism  consists  of  a  ring  concentric  with 
the  axis  of  the  armature  and  attached  to  the  field  frame. 
The  simultaneous  shifting  of  the  brushes  is  accomplished 
by  the  turning  of  the  hand  wheel.  These  generators  are 
rated  on  the  basis  that  after  a  continuous  full-load  run  of 
24  hours  the  temperature  elevation  of  no  part  of  the  ma- 
chine will  rise  more  than  35°  above  the  engine-room  tem- 
perature. A  subsequent  increase  of  50  per  cent  full  load 
for  two  hours  will  cause  no  more  than  55°  C.  temperature 
elevation  over  the  surrounding  air. 


Fig.  IDS. 

A  belt -driven,  4OO-K.W.,  375  rev.  per  min.  generator 
manufactured  by  the  Western  Electric  Company  is  shown  in 
Fig.  1 02.  The  pole  pieces  are  of  laminated  sheet  steel,  and 
are  cast  into  the  circular  yoke,  thus  insuring  good  magnetic 
joints.  In  the  larger  machines  the  frames  are  divided  ver- 


GENERATORS. 


177 


tically,  a  construction  that  permits  of  easy  access  to  the 
armature  without  necessitating  the  use  of  heavy  hoisting 
apparatus.  Slide  rails  are  provided  upon  which  the  ma- 
chines may  be  moved  by  means  of  a  screw  in  order  to 
tighten  the  belt.  Alignment  is  maintained  by  tongues  in  the 
base  of  the  machine  which  fit  into  grooves  in  the  slide  rails. 

The  Allis-Chalmers  Company  manufactures  generators 
of  the  belted  "H  "  type  in  sizes  of  from  7.5  to  500  K.W. 
for  voltages  of  120,  240  and  500  volts,  and  engine-type 
generators  from  12  to  1000  K.W.  The  field  poles  of 
these  machines  are  made  up  of  laminated  steel  stampings 


Fig.  103. 

of  the  shape  shown  in  Fig.  103.  In  assembling  these 
punchings  to  form  the  poles,  they  are  alternately  reversed 
with  respect  to  one  side.  Thus,  the  face  of  the  pole  for  a 
short  depth  contains  but  one-half  as  much  iron  as  the  main 
body  of  the  pole.  This  results,  under  normal  excitation, 
in  a  saturated  pole  face.  It  has  the  same  effect  in  pre- 
venting distortion  of  the  field  under  the  influence  of  arma- 
ture reaction  as  saturation  of  the  teeth  of  the  armature 
core.  The  teeth  can  therefore  be  operated  at  a  smaller 
magnetic  flux  density.  The  hysteresis  losses  in  the  teeth 
can  accordingly  be  made  smaller.  The  thinness  of  the 
stampings,  and  the  ideally  perfect  lamination  of  the  pole 
face,  permit  the  use  of  a  smaller  ratio  of  tooth  width  to  slot 
width,  without  the  excessive  eddy  current  loss  in  the  pole 


178  DYNAMO   ELECTRIC   MACHINERY. 

face  which  would  occur  in  other  machines.  The  possibility 
of  using  narrow  teeth  results  in  a  reduction  of  the  induc- 
tances of  the  armature  coils.  This  facilitates  effective  com- 
mutation. 


Fig.  104. 


Fig.  104  shows  a  35O-K.W.  engine-type  generator  made 
by  the  Westinghouse  Electric  and  Manufacturing  Company. 


GENERATORS. 


1/9 


Some  of  the  important  features  of  this  design  are  the  use 
of  conductor  retaining  wedges  in  the  armature  slots,  the 
arrangement  of  the  series  field  coil  connections,  removable 
pole  pieces,  and  the  arrangement  of  armature  equalizer 
rings  and  of  the  brush-holder  shifting  device. 


Fig.  105. 


Fig.  105  depicts  a  General  Electric  Company  generator 
direct  coupled  to  a  steam  turbine,  and  mounted  on  a  com- 
mon bedplate.  The  cut  shows  the  generating  unit  as 
semitransparent  so  as  to  reveal  the  interior  parts. 

The  field-magnet  frame  of  a  6-pole  single-coil  type  of 
Lundell  generator  with  its  field  coil  in  place  is  shown  in 
Fig.  1 06.  The  frame  is  divided  in  a  vertical  plane  which 
is  perpendicular  to  the  axis  of  the  armature. 


i8o 


DYNAMO   ELECTRIC    MACHINERY. 


Fig.  106. 

81.  Three-Wire  Generators.  —  The  adoption  of  three- 
wire  systems  of  electrical  distribution,  particularly  for  light- 
ing, is  due  to  the  saving  of  copper  in  the  line  conductors. 
The  standard  voltage  for  incandescent  lamps  is  about  1 1  o 
volts.  At  this  pressure  large  conductors  must  be  em- 
ployed on  long  lines  in  order  to  maintain  a  fairly  constant 
voltage  at  the  lamps  as  the  load  changes.  By  doubling  the 
voltage  across  the  mains  and  connecting  the  lamps  thereto 
so  that  they  are  joined  by  pairs  in  series  with  each  other, 
only  one-fourth  as  much  copper  need  be  used,  since  the 


GENERATORS.  l8l 

same  power  is  transmitted  at  half  the  current,  and  for  the 
same  permissible  drop  the  conductors  need  be  but  half  as 
large.  But,  in  order  that  each  lamp  may  be  operated  in- 
dependently of  the  others,  a  balancing  wire  or  neutral 
wire  must  be  provided,  and  this  is  usually  of  the  same 
size  as  the  other  conductors.  Therefore  the  weight  of  con- 
ductors on  a  three-wire  system  will  be  J  +  \  =  f  as  much 
as  on  a  two-wire  system. 

The  introduction  of  a  neutral  wire  involves  the  genera- 
tion of  the  total  E.M.F.  in  two  parts  so  that  this  neutral 
wire  may  constitute  a  common  conductor  for  the  two  com- 
ponent voltages.  To  obtain  this 
condition,  two  generators  may  be 
connected  as  in  Fig.  107,  but  as 
this  signifies  additional  expense, 
various  other  methods  have  been 
adopted  to  obtain  three-wire  dis- 
tribution. These  practical  methods  Flg* I07' 
employ:  (a)  dynamotors,  §  no,  which  have  two  armature 
windings  upon  the  same  core  connected  to  two  separate 
commutators,  and  connected  in  the  same  manner  as  two 
individual  generators;  (b)  storage  batteries,  §  113,  of  suffi- 
cient number  of  cells  connected  between  the  two  outside 
wires,  the  neutral  wire  connecting  with  the  middle  point  of 
the  battery;  (c)  balancers,  §  1 1 1,  which  are  two  mechani- 
cally coupled  dynamos  connected  across  the  outside  wires, 
one  of  which,  if  the  system  be  unbalanced,  will  run  as  a 
motor  and  drive  the  other  as  a  generator  which  supplies 
energy  to  the  more  heavily  loaded  side;  (d)  three-brush 
dynamos,  and  (e)  three-wire  generators. 

The  total  voltage  of  a  generator  could  be  divided  into 
two  parts  by  placing  a  brush  midway  between  the  positive 


1 82 


DYNAMO   ELECTRIC   MACHINERY. 


and  negative  brushes  ;  but,  for  satisfactory  current  collec- 
tion, the  coil  short-circuited  by  this  additional  brush  must 
lie  in  a  weak  magnetic  field.  Such  an  arrangement  was 
developed  by  Dettmar,  and  is  shown  in  Fig.  108.  This 

illustrates  a  four-pole  field-magnet 
frame  wound  as  a  bipolar  machine 
with  two  adjacent  north  poles  and 
two  adjacent  south  poles.  The 
yokes  of  such  machines  between 
oppositely  named  poles  must  be 
of  sufficient  cross-section  to  carry 
the  total  flux  per  pole  at  a  reason- 
able flux  density.  The  tendency 
Fig-  I08>  of  the  armature  current  is  to  crowd 

the  flux  toward  the  leading  poles,  thus  resulting  in  a  some- 
what greater  voltage  between  the  positive  terminal  and 
the  neutral  wire  than  between  the  latter  and  the  negative 
terminal. 


Fig.  109. 


The  three-wire  generator  designed  by  Dobrowolsky  is  well 
adapted  for  three-wire  supply  circuits.  Points  of  the  arma- 
ture winding  at  distances  from  one  another  equal  to  twice 
the  pole  pitch  are  connected  to  one  slip  ring  and  the  inter- 


GENERATORS. 


183 


mediate  points  are  connected  to  another  slip  ring.  Brushes 
bearing  upon  these  collector  rings  connect  with  the  ends 
of  a  coil  wound  on  an  iron  core,  called  a  reactor,  as  shown 
in  Fig.  109.  The  middle  point  of  the  reactor,  D,  connects 
to  the  neutral  wire  of  the  system,  the  outside  wires  being 


Fig.  no. 


connected  to  the  brushes  B.  The  reactor  has  a  low  re- 
sistance, but  a  large  inductance.  The  electromotive  force 
across  the  terminals  C  is  an  alternating  E.M.F. ;  and,  be- 
cause of  the  large  inductance  of  the  coil,  very  little  current 


184  DYNAMO   ELECTRIC   MACHINERY. 

flows  through  it  when  the  loads  on  the  two  sides  of  the 
system  are  equal.  If  the  system  be  unbalanced,  the  cur- 
rent flowing  in  the  neutral  wire,  since  it  is  direct  current, 
will  suffer  little  impedance  in  passing  through  the  reactor. 

A  I5O-K.W.,  6-pole  General  Electric  Company  genera- 
tor, provided  with  two  slip  rings  for  connection  to  a  reactor 
as  just  described,  is  shown  in  Fig.  no.  These  machines 
are  usually  wound  for  250  volts,  so  that  125  volts  can  be 
obtained  on  either  side  of  the  three-wire  system.  They 
may  be  flat-  or  over-compounded  to  compensate  for  line 
drop. 

Three-wire  generators  having  a  single  slip  ring  for  con- 
nection to  the  neutral  wire  are  manufactured  by  the  Burke 
Electric  Company.  The  reactor  forms  a  part  of  the  arma- 
ture and  revolves  with  it,  the  middle  point  of  the  reactor 
being  connected  to  the  slip  ring. 

Three-wire  generators  are  extensively  used  in  isolated 
plants  for  electric  lighting  and  light  power  service. 

82.  Homopolar  Dynamos. — A  type  of  direct-current 
generator  in  which  the  armature  conductors  move  in  a  uni- 
directional and  uniform  magnetic  field,  and  therefore  have 
induced  in  them  electromotive  forces  of  constant  direction 
and  magnitude,  is  known  as  the  Jiomopolar  dynamo,  some- 
times also  as  the  acyclic  or  unipolar  dynamo.  Fig.  in 
shows  a  cross-section  of  a  simple  machine  of  this  type,  with 
one  conductor,  A,  connected  to  two  slip  rings  B.  The 
magnetic  field  of  the  generator  is  set  up  by  the  current 
flowing  in  the  field  coils  C  ;  the  paths  of  the  lines  of  force 
being  represented  by  the  dotted  lines.  The  current  is  led 
from  the  machine  by  means  of  brushes  which  slide  upon 
the  slip  rings  and  are  connected  to  wires  projecting  through 
apertures  in  the  field-magnet  yoke. 


GENERATORS. 


I85 


Single-conductor  homopolar  dynamos  are  suitable  for 
supplying  a  large  current  at  low  voltage,  and  even  then  the 
magnetic  flux  traversing  the  air-gap  must  be  large  and 
the  armature  must  be  run  at  high  speed.  As  there  is  little 
demand  for  such  low- voltage  generators,  homopolar  machines 


Fig.  in. 

for  practical  purposes  must  be  designed  to  generate  higher 
voltages.  This  may  be  accomplished  by  increasing  the 
number  of  armatures  mounted  together  to  form  one  ma- 
chine, or  by  employing  several  conductors  insulated  from 
one  another  and  connected  in  series.  The  end  of  one  con- 
ductor must  be  joined  to  the  beginning  of  another,  but  this 
end  connection  must  not  cut  the  lines  of  force,  otherwise 
the  resultant  E.M.F.  would  be  zero.  Consequently  the 
end  connections  must  be  stationary,  and  this  means  that 
two  slip  rings  must  be  provided  for  each  conductor.  Limi- 
tations to  increasing  the  number  of  conductors  are  the 
available  space  for  the  slip  rings  and  the  increased  brush 
friction. 


1 86 


DYNAMO   ELECTRIC    MACHINERY. 


The  electromotive  force  generated  by  a  multi-conductor 
homopolar  dynamo  is 

y 

60 


volts, 


where       N  =  number  of  conductors  in  series, 

4>  =  total  flux  entering  armature, 
and  V  =  rev.  per  min. 

The  armature  of  a  3OO-K.W.,  5oo-volt,  3000  rev.  per 
min.,  turbine-driven  homopolar  generator  is  shown  in  Fig. 
112.  It  consists  of  12  copper  conductors  mounted  on  a 


Fig.  112. 

cast-steel  core,  the  ends  of  the  conductors  being  connected 
to  12  cast-steel  slip  rings  at  either  end  of  the  armature. 
One  copper  brush  is  provided  for  each  ring,  and  access  is 
obtained  thereto  through  apertures  in  the  cast-steel  field- 
magnet  frame.  Compounding  is  effected  by  utilizing  the 
M.M.F.  of  the  currents  in  the  stationary  leads  which  are 
connected  to  the  brushes,  or  that  of  the  currents  in  the 
slip  rings  from  the  connection  points  to  the  brushes,  to  aid 
the  M.M.F.  of  the  field  current.  The  degree  of  com- 
pounding may  be  varied  by  shifting  the  brushes.  Homo- 
polar  generators  may  be  separately-  or  self-excited. 


GENERATORS. 


I87 


As  the  air-gap  of  homopolar  machines  may  be  very 
small,  the  field  current  need  not  be  great  in  order  to  set 
up  the  required  magnetic  flux  100, 
through  the  armature.  This 
fact  indicates  a  low  excitation 
loss.  There  are  practically  no  |  8° 
iron  losses  in  this  type  of  jt  70 
generator  because  of  the  con-  jjj 
stancy  of  flux  density.  The  £  6° 
brush  losses,  however,  are  so 
large.  The  efficiency  curve 
of  the  3OO-K.W.  generator 
is  shown  in  Fig.  113.  The  Fig.  113. 

voltage  regulation  is  from  6  to  1 2  per  cent. 


60         76         100 

PERCENT   FULL   LOAD 


126 


Constant-Current  Generators. 

83.    Characteristic  Curves  of  Series-Wound  Generators. 

—  Fig.  114  shows  the  characteristic  curves  of  a  15  K.W. 
series-wound  generator  at  a  speed  of  1000  rev.  per  min. 
Curve  E  indicates  the  terminal  voltage  of  the  machine 
when  delivering  various  currents,  and  is  called  the  external 
characteristic.  This  curve  shows  that  the  E.M.F.  of  the 
generator  at  first  increases  in  proportion  to  the  current 
output,  but  as  the  load  increases  the  resistance  drop  of  the 
field  and  armature  windings  and  armature  reaction  cause 
the  curve  to  bend  back.  This  is  also  due  to  the  fact  that, 
as  the  magnetic  circuit  approaches  saturation,  the  magnetic 
flux  does  not  increase  proportionally  to  the  increase  of  field 
current.  To  obtain  the  external  characteristic  of  a  series- 
wound  generator  experimentally  the  machine  is  run  at  a 
definite  and  constant  speed  and  observations  of  terminal 


i88 


DYNAMO   ELECTRIC    MACHINERY. 


voltage  and  current  output  are  made  at  different  loads.  At 
constant  load  the  terminal  voltage  will  vary  directly  with 
the  speed. 

The  total  characteristic  of  a  series-wound  generator  may 
be  obtained  from  the  external  characteristic  and  the  resist- 
ance drop  of  the  windings,  §  74.  Thus  in  Fig.  114,  curve 


AMPERES 

Fig.  114. 


120 


EI  is  plotted  by  adding  the  ordinates  of  the  curves  of  ter- 
minal voltage  and  resistance  drop.  The  curves  of  both  R 
and  Et  start  above  zero  because  of  residual  magnetism  in 
the  cores  of  the  field  magnets. 

The  total  characteristic  resembles  the  magnetization 
curve  of  series-wound  generators.  The  latter  is  a  curve 
which  shows  the  terminal  voltage  of  a  machine  at  no  load 
for  different  values  of  field  current.  The  difference  exist- 


GENERATORS. 


189 


ing  between  these  curves  is  due  solely  to  the  demagnetizing 
effect  of  the  armature  current  on  the  magnetic  circuit. 

84.  Power  Lines.  —  Where  volts  and  amperes  are  used 
as  ordinates  and  abscissae,  lines  can  be  drawn  connecting 
points  of  constant  product  of  the  two,  representing  watts 
or  power.  Fig.  115  shows  such  lines  drawn  for  one,  two, 


20  30 

AMPERES 


Fig.  115. 

and  three  kilowatts.  If  E  be  the  external  characteristic  of 
a  dynamo,  then  the  curves  make  it  apparent  that  the 
machine  cannot  generate  3  K.W.,  but  that  for  most  values 
under  3  K.W.  there  will  be  two  loads  under  which  the 
generator  can  run  and  yield  the  same  voltage. 

85.  Series-Wound  Generators.  —  The  advantage  of  con- 
stant-current distribution  for  arc  lighting  lies  in  the  saving 
of  conductor  material.  In  this  system,  as  the  load  increases 
the  voltage  must  increase  a  corresponding  amount,  so  that 


190  DYNAMO   ELECTRIC   MACHINERY. 

the  current  flowing  will  be  unchanged.  An  ordinary  series 
arc  lamp,  as  it  is  trimmed  and  adjusted  for  general  use,  re- 
quires between  45  and  50  volts  to  force  its  rated  current 
through  it.  A  generator  supplying  a  circuit  of  say  2000 
candle-power  lamps  with  n  such  lamps  in  the  circuit  must 
be  capable  of  generating  a  constant  current  of  9.8  amperes. 
It  must  be  able  to  regulate  its  pressure  between  the  limits 
of  50  and  50  ;/  volts.  This  is  necessary  in  order  that  it  may 
operate  all  the  lamps  or  any  part  of  the  whole  number  at 
proper  illumination. 

The  current  of  an  arc-light  machine  must  not  exceed  nor 
fall  below  its  normal  value,  no  matter  how  suddenly  the 
load  is  varied  ;  for  the  slightest  change  affects  the  intensity 
of  the  light  at  the  lamps.  It  is  obvious  that  some  mechan- 
ical device  could  be  applied  to  an  ordinary  shunt-wound 
generator  to  cause  it  to  give  constant  current,  either  by 
changing  the  position  of  the  brushes  or  by  varying  the 
ampere-turns  of  the  field  coils.  However,  any  such  device 
would  be  slow  of  operation,  and  a  sudden  short-circuit 
would  cause  a  destructive  current  to  flow  before  the  reg- 
ulator completed  its  action.  It  is  therefore  necessary  to 
rely  on  the  armature  reactions  for  regulation,  since  they 
vary  simultaneously  with  the  current.  All  successful  con- 
stant-current machines  are  constructed  on  this  principle. 
The  machine  is  designed  with  a  magnetic  field  of  great 
intensity,  the  armature  reactions  are  very  great,  and  thus 
the  total  flux  effective  in  producing  E.M.F.  is  reduced. 
A  slight  increase  of  current  in  the  armature  materially  in- 
creases the  armature  reactions.  The  effective  flux  is  thus 
reduced,  and  the  pressure  falls  until  the  current  returns  to 
its  normal  value.  Thus  the  machine  is  completely  and  in- 
stantly self-regulating.  The  field  coils  are  series  wound  on 


GENERATORS.  igi 

all  arc-light  generators,  and  the  cores  of  the  field  magnets 
are  worked  at  a  very  high  magnetic  density,  since  the  mag- 
nets are  then  less  sensitive  to  slight  changes  in  the  mag- 
netizing force.  In  commercial  machines  the  densities  in 
the  field  cores  are  from  17,000  to  18,000  lines  per  square 
centimeter  for  wrought  iron  or  steel,  and  from  9000  to 
1 1,000  lines  for  cast  iron. 

In  the  armature  high  magnetic  density  is  also  required 
to  prevent  a  sudden  rise  of  voltage  when  the  circuit  is 
broken.  With  no  current  in  the  armature,  the  total  mag- 
netomotive force  of  the  field  magnets  would  be  effective  in 
producing  E.M.F.,  and  a  destructive  rise  of  pressure  would 
result,  since  the  total  M.M.F.  of  the  field  magnets  is  much 
greater  than  the  normal  effective  M.M.F.  But  a  high 
magnetic  density  in  the  armature  core  leaves  the  latter 
incapable  of  receiving  such  an  increase  of  flux,  and  there- 
fore destructive  voltages  are  avoided.  In  practice  the 
armature  core  is  designed  to  have  a  density  of  from  15,000 
to  20,000  lines  per  square  centimeter  at  its  minimum  cross- 
section. 

A  consideration  of  the  foregoing  theory  of  regulation 
shows  that  the  following  conditions  should  obtain  more  or 
less  completely  in  a  successful  constant-current  generator : 
(a)  since  the  current  is  small,  there  must  be  a  great  number 
of  armature  turns ;  (b]  the  magnetic  field  of  the  machine 
must  be  much  distorted ;  (c)  the  path  of  the  lines  of  force 
of  the  field  coils  must  be  long  and  of  small  area,  so  the 
M.M.F.  cannot  be  readily  changed  ;  (d)  the  path  of  the 
lines  of  force  due  to  armature  magnetization  must  be  short 
and  of  great  area,  so  that  the  M.M.F.  of  the  armature  will 
change  with  the  slightest  change  of  current ;  and  (e)  the 
pole  pieces  must  be  worked  at  a  high  flux  density. 


192  DYNAMO    ELECTRIC   MACHINERY. 

Evidently  extreme  difficulty  is  found  in  so  designing  the 
different  parts  of  the  machine  as  to  give  proper  considera- 
tion to  each  of  the  conditions  and  yet  produce  a  machine 
that  will  regulate  for  constant  current  at  all  loads.  This 
leads  to  the  introduction  of  automatic  mechanical  devices 
for  aiding  in  the  regulation.  These  devices  must  not  be 
considered  as  being  the  sole  regulators,  for  in  every  case 
they  are  secondary  to  the  natural  self-regulating  tendency 
of  the  armature.  In  general  they  regulate  for  the  gradual 
and  greater  changes  of  load,  while  the  armature  reactions 
take  care  of  the  smaller  and  more  sudden  fluctuations. 

There  are  two  general  systems  of  regulating  arc-light 
dynamos.  The  first  method  is  to  cause  the  machine  to  de- 
velop an  E.M.F.  in  excess  of  that  required  for  the  load,  and 
then  to  collect  an  E.M.F.  just  sufficient  for  the  load  in 
hand.  This  is  done  by  shifting  the  brushes  from  the  neu- 
tral plane  (§52).  In  a  closed-coil  armature  this  causes  a 
counter  pressure  to  be  developed  in  those  conductors  lying 
between  the  commutating  plane  and  a  similar  plane  in  the 
other  direction  making  an  equal  angle  with  the  neutral 
axis.  This  reduces  the  pressure  to  the  desired  amount. 
In  an  open-coil  armature  the  brushes,  when  in  the  maxi- 
mum position,  connect  to  the  circuit  those  coils  of  the 
armature  which  at  that  instant  have  the  maximum  E.M.F. 
generated  in  them.  By  shifting  the  brushes  either  way, 
coils  can  be  connected  to  the  circuit  which  have  some 
E.M.F.  lower  than  the  total  E.M.F.  generated  in  them,  and 
the  amount  of  shifting  regulates  the  pressure  on  the  line. 

The  second  method  of  arc-light  dynamo  regulation  is  to 
vary  the  magnetizing  force  in  the  field  magnets  just  enough 
to  put  the  required  pressure  on  the  line.  Since  the  mag- 
netizing force  is  dependent  on  the  ampere-turns  of  the  field 


GENERATORS.  193 

coils,  it  can  be  varied  either  by  cutting  out  or  short-circuit- 
ing some  of  the  turns  or  by  changing  the  current  in  them 
by  means  of  a  variable  resistance  which  is  shunted  across 
the  field  terminals.  In  practice  both  these  methods  have 
been  used. 

Whether  regulation  is  effected  by  changing  the  position 
of  the  brushes,  or  by  changing  the  field  excitation,  sparking 
will  occur  at  the  points  of  collection  of  the  current  if  means 
are  not  provided  to  avoid  it.  Sparkless  collection  could  be 
obtained  were  the  magnetic  field  perfectly  uniform  all  around 
the  armature.  In  general  this  condition  is  impracticable, 
since  it  requires  almost  the  whole  armature  to  be  covered 
by  the  pole  faces,  and  it  requires  the  density  in  the  gap 
beneath  them  to  be  uniform.  Considerations  of  magnetic 
leakage  and  armature  reaction  render  almost  impossible  the 
satisfying  of  these  conditions.  Another  and  more  practical 
method  is  to  employ  for  current  collection  at  one  terminal 
of  the  machine  two  brushes  connected  in  parallel.  These 
are  moved  in  opposite  directions,  thus  giving  the  effect  of  a 
single  brush  of  varying  circumferential  contact,  the  center 
of  which  can  always  be  kept  in  the  neutral  plane.  This 
device  avoids  excessive  sparking,  and  is  used  quite  success- 
fully in  practice.  There  is,  however,  some  question  as  to 
the  advisability  of  resorting  to  it. 

86.  The  Brush  Machine.  —  Fig.  116  shows  a  standard 
i6o-light  Brush  arc-light  generator,  made  by  the  General 
Electric  Company.  The  armature  revolves  between  the 
pole  faces  of  two  sets  of  field  magnets.  Like  poles  are  op- 
posed to  each  other.  The  flux,  therefore,  takes  a  path  out 
of  the  opposing  pole  faces  into  the  armature  core,  and  then 
circumferentially  through  the  core  and  out  into  the  next 
pair  of  opposing  pole  faces. 


194 


DYNAMO   ELECTRIC   MACHINERY. 


The  armature  is  of  the  open-coil  type  and  consists  of  a 
number  of  coils  or  bobbins  placed  on  a  ring  core  of  greater 
radial  depth  than  breadth,  and  the  pole  faces  cover  the  sides 


Fig.  116. 

instead  of  the  circumference.  The  individual  bobbins  are 
protected  by  an  insulating  box,  but  are  not  surrounded  by 
any  masses  of  metal.  This  fact,  together  with  the  fact 
that  the  armature  is  of  such  shape  as  to  cause  great  air 


GENERATORS.  195 

disturbances,  insures  exceptionally  good  ventilation  of  the 
armature.  This  machine  is  of  relatively  slow  speed,  the 
larger  sizes  running  at  only  500  rev.  per  min. 

At  a  given  instant  of  time,  the  different  coils  on  the 
moving  armature  have  E.M.F.'s  of  widely  different  magni- 
tudes induced  in  them.  The  commutator,  Fig.  117,  is  so 


Fig.  117. 

designed  that  it  connects  the  coils  of  highest  E.M.F.  in 
series  with  one  another  to  the  external  circuit*  and  con- 
nects the  coils  of  medium  E.M.F.  in  multiple  with  one 
another  to  the  external  circuit,  while  those  of  smallest 
E.M.F.  are  cut  out  entirely  from  the  circuit. 

The  bearings  are  self-lubricated  by  means  of  rings. 
Since  the  poles  are  on  the  sides  of  the  armature,  side  play 
in  the  bearings  must  be  prevented.  The  commutator  end 
of  the  shaft  is  turned  with  thrust  collars  which  are  engaged 
by  corresponding  annular  recesses  in  the  brasses. 

Voltage  regulation  on  these  machines  is  effected  by  a 
variable  resistance  in  shunt  with  the  field  coils  ;  and  as  the 
field  current  is  changed  the  position  of  the  brushes  is  also 
changed,  not  to  collect  current  at  a  lower  voltage,  as  de- 


196 


DYNAMO   ELECTRIC   MACHINERY. 


scribed  in  §85,  but  to  obtain  sparkless  collection.  These 
two  operations  are  performed  by  a  regulator,  shown  in  Fig. 
1 1 8,  which  is  attached  directly  to  the  frame  of  the  machine. 
The  mechanism  consists  of  a  rotary  oil-pump  driven  by  a 
belt  from  the  armature  shaft,  a  balance  valve  of  the  piston 


Pig,  118. 

type,  and  a  rotary  piston  in  a  short  cylinder,  which  is 
directly  connected  to  an  arm  moving  over  the  contacts  of 
the  field-shunt  rheostat.  The  valve  is  operated  by  a  lever 
actuated  by  a  controlling  electro-magnet  which  is  energized 
by  the  whole  generator  current.  At  normal  current  the 
valve  is  centrally  placed,  and  the  oil  from  the  pump  flows 
around  the  overlapping  ports  into  the  reservoir  without 


GENERATORS. 


197 


effect.  If  the  current  rises  above  the  normal,  the  armature 
of  the  controlling  magnet  is  attracted,  the  balance  valve 
moves  up,  and  oil  enters  the  cylinder,  moving  the  rotary 
piston  in  a  clockwise  direction.  The  shaft  of  this  piston 
moves  the  arm  of  the  rheostat,  cutting  out  resistance  and 
thus  lowering  the  field  exciting  current.  At  the  same 


Fig.  119. 

time  a  pinion  on  the  shaft,  seen  in  Fig.  119,  actuates  a 
rocker  arm  which  moves  the  brush  holders  to  a  position 
such  that  the  collection  of  current  by  the  brushes  will  be 
sparkless.  When  the  current  returns  to  its  normal  value 
the  adjusting  spring  returns  the  lever  and  balance  valve  to 
the  central  position.  If  the  current  falls  below  normal 
value,  these  operations  are  reversed.  It  is  claimed  for  this 


198 


DYNAMO   ELECTRIC   MACHINERY. 


regulator  that  it  can  bring  the  current  back  to  normal  from 
a  complete  short-circuit  in  from  3jto  4  seconds.  The  ten- 
sion of  the  adjusting  spring  can  be  regulated  from  the  out- 
side of  the  dust-proof  case  by  a  hard  rubber  knob. 

87.    The  Excelsior  Arc-Light  Generator.  —  This  machine, 
Fig.   120,  is  a  closed-coil  ring-armature  generator,  having 

pole  faces  that  cover  both 
the  sides  and  the  circum- 
ference of  the  armature. 
The  interesting  feature  of 
this  machine  is  the  method 
of  regulation.  The  proper 
voltage  is  supplied  to  the 
line  by  using  both  methods 
of  control  in  conjunction  ; 
that  is,  sections  of  the  field 
windings  are  cut  in  or  out 
of  the  circuit,  and  at  the 
same  time  the  position  of 
the  brushes  is  shifted.  The 
proper  motion  of  the  field 
regulator  arm  and  of  the 
brush  holder  is  obtained  by 
means  of  a  small  motor 
from  the  main  magnets  of  the 
machine.  This  motor  is  operated  by  a  device  shown 
in  Fig.  121.  The  whole  device  is  inserted  in  series  with 
one  of  the  mains  from  the  generator.  The  right-hand 
lever  is  of  insulating  material,  with  the  contact  blocks  a 
and  b  properly  placed  upon  it.  The  left-hand  lever  is  of 
conducting  material,  and  is  capable  of  being  attracted  by 
the  electro-magnet  which  is  excited  by  the  main  current. 


Fig.  120. 


whose  field  is  "  sneaked 


GENERATORS. 


199 


The  magnet  and  spring  are  so  adjusted  that  when  the  nor- 
mal current  is  flowing,  both  a  and  b  are  in  contact  with  the 
left  lever,  and  the  current  flows  in  the  three  shunt  paths, 
R,  Rv  and  R2.  There  will  be  no  current  in  the  armature 
of  the  regulating  motor,  since  the  potential  at  brush  x  is 


From  Dynamo 


Fi;.  121. 


equal  to  the  potential  at  brush  y.  If  now  the  line  current 
becomes  too  strong  the  magnet  attracts  the  left  lever  to 
it  and  the  contact  at  a  is  broken.  Immediately  the  current 
flowing  through  b  divides  at  the  brush  x,  part  going  through 
R2  and  part  through  the  motor  armature  and  R^.  The 
motor  will  then  revolve  in  a  given  direction,  and  by  simple 


200  DYNAMO  ELECTRIC   MACHINERY. 

mechanical  devices  will  cut  out  sections  of  the  field  wind- 
ings, and  will  shift  the  brushes  until  the  normal  current  is 
flowing,  when  contact  is  again  made  at  a  and  the  control- 
ling motor  stops.  If  the  line  current  drops  below  normal, 
the  spring  pulls  the  lever  away  from  the  magnet  and  the 
contact  at  b  is  broken.  Part  of  the  current  then  flows  from 
y  to  x  through  the  motor  armature.  It  therefore  revolves 
in  a  direction  opposite  to  that  which  it  had  before.  The 
brushes  on  the  dynamo  are  shifted  back  again,  and  more 
sections  of  field  winding  are  put  into  circuit. 

When  the  current  is  broken  at  a  or  b,  there  is  no  serious 
sparking,  since  there  are  always  two  circuits  in  shunt  with 
the  break.  The  whole  current  of  the  dynamo  does  not 
exceed  ten  amperes  ;  and  the  resistances  R,  R^  and  R2  are 
so  proportioned  that  only  a  small  portion  of  this  current 
flows  through  a  or  b. 

In  practice  the  levers  and  the  magnet  are  mounted  on 
the  wall  or  the  switchboard,  while  the  regulating  motor  is 
mounted  on  the  dynamo  frame. 

88.  The  Thomson-Houston  Dynamo. — The  Thomson- 
Houston  arc-light  generator  is  of  a  type  entirely  different 
from  the  other  machines  here  described,  not  only  in  appear- 
ance, but  also  in  method  of  armature  winding  and  of  regu- 
lation. A  view  of  this  machine  is  given  in  Fig.  122.  Each 
field  coil  has  for  its  core  an  iron  tube,  flanged  exteriorly  at 
each  end  to  form  a  recess  for  the  windings,  and  fitted  at 
the  armature  end  with  a  concave  iron  piece  that  surrounds 
part  of  the  armature.  This  tube,  with  the  flanges  and  the 
cup-shaped  end,  is  cast  in  one  piece.  The  farthermost 
flange  of  each  field  core  is  bolted  to  a  number  of  wrought- 
iron  connecting-rods  which  hold  the  magnets  in  place,  pro- 
tect the  field  windings,  and  take  the  place  of  the  yoke  of 


GENERATORS. 

other  machines  in  completing  the  magnetic  circuit.  The 
magnets  are  mounted  on  a  frame,  including  legs  and  bear- 
ing supports  for  the  armature  shaft. 

The  armatures  of  the  older  machines  of  this  type  are 
spheroidal  in  shape,  while  the  more  recent  ones  have  ring 


Fig.  122. 

armatures ;  these  are  more  readily  repaired  or  rewound. 
The  winding  of  either  form  of  armature  is  peculiar  in  that 
only  three  coils  are  employed,  set  with  an  angular  displace- 
ment from  one  another  of  120  degrees.  The  inner  ends 
of  the  three  coils  are  joined  to  each  other,  and  are  not  at- 
tached to  any  other  conductor,  an  arrangement  unique  in 


202  DYNAMO   ELECTRIC   MACHINERY. 

direct-current  dynamos.  The  outside  ends  are  connected 
to  the  segments  of  a  three-bar  commutator,  from  which  the 
current  is  collected  by  four  copper  brushes  connected  in 
multiple. 

Regulation  is  obtained  by  shifting  the  brushes  in  the 
following  manner.  Fig.  123  shows  the  two  possible  rela- 
tions between  brushes  and  commutator  that  may  exist  at 
any  instant.  Both  brushes  of  each  set  may  rest  on  one 
commutator  bar,  or  the  brushes  of  one  set  may  span  the 
gap  between  the  other  two  bars.  These  conditions  are  re- 
peated three  times  at  each  brush  for  each  revolution.  If 


Fig.  123. 

the  dotted  line  shows  the  position  where  the  maximum 
E.M.F.  is  generated  in  the  coils,  then  in  Fig.  123  a  the  two 
most  active  coils  are  connected  in  series  with  the  outside 
circuit,  while  the  coil  near  the  position  of  least  activity  is 
out  of  circuit.  In  Fig.  123  b  the  two  less  active  coils  are 
in  multiple  with  themselves  and  in  series  with  the  most 
active  coil  and  the  external  circuit.  In  practice  the  brushes 
of  a  set  are  60  degrees  apart,  leaving  1 20  degrees  between 
the  leading  brush  of  one  set  and  the  following  brush  of  the 
other  set ;  and  since  1 20  degrees  is  the  angular  measure 


GENERATORS. 


203 


of  the  length  of  a  commutator  bar,  there  is  no  coil  out  of 
circuit  at  normal  load,  two  being  always  in  parallel  and  in 
series  with  the  third.  If  the  current  rise  above  the  normal 
the  leading  brushes  move  a  small  angle  forward,  while  the 
following  brushes  recede  through  three  times  that  angle. 
This  will  shorten  the  time  that  a  single  coil  gives  its  whole 
E.M.F.  to  the  circuit,  and  will  place  it  more  quickly  in  par- 
allel with  a  comparatively  inactive  coil.  But  such  a  move- 
ment will  reduce  the  angular  distance  between  the  nearest 
brushes  of  the  opposite  sets  to  less  than  1 20  degrees,  hence 
the  machine  will  be  short-circuited  six  times  per  revolution, 
since  one  brush  of  each  set  will  touch  one  segment  of  the 
commutator  at  the  same  time.  If  the  current  in  the  line 
falls  below  normal,  then  the  brushes  close  together,  and 
the  time  that  a  coil  is  in  series  is  lengthened,  and  the  time 
that  it  is  in  parallel  with  an  inactive  one  is  lessened. 


field 


The  arrangement  for  moving  the  brushes  is  shown  in 
Fig.  124.  The  leading  brushes  are  shifted  forward  on  an 
increase  of  current  merely  to  help  avoid  sparking.  The 


204  DYNAMO   ELECTRIC   MACHINERY. 

brushes  are  moved  by  levers  actuated  by  a  series  magnet 
A.  This  magnet  is  normally  short-circuited  by  the  by- 
pass circuit.  On  an  undue  rise  of  current  this  circuit  is 
broken  by  the  series  magnet  B.  A  then  becomes  more 
powerful,  and  the  levers  separate  the  brushes.  While  the 
machine  is  in  operation  the  circuit-breaker  C  is  constantly 
vibrating,  the  brushes  adjusting  to  suit  the  load.  A  high 
carbon  resistance  is  shunted  across  C  to  prevent  sparking 
at  that  point. 

As  might  be  expected,  with  but  three  parts  to  the  com- 
mutator and  collection  made  with  small  regard  to  the 
neutral  point,  the  sparking  of  this  machine  is  such  as  speed- 
ily to  ruin  the  commutator  and  the  brushes,  if  means 
are  not  taken  to  suppress  it.  A  rotary  blower  is  mounted 
on  the  shaft,  and  is  arranged  to  give  intermittent  puffs  of 
air,  which  at  the  right  moment  blow  out  the  spark.  The 
insulation  between  the  segments  is  air,  considerable  gap 
being  left  between  them,  and  through  these  gaps  the  sparks 
are  blown. 

89.  Western  Electric  Arc -Light  Dynamo.  —  Fig.  125 
represents  a  Western  Electric  Company  generator,  which 
is  regulated  by  means  of  shifting  the  brushes.  The  brush 
and  rocker  are  connected  by  means  of  a  link  and  a  ball-and- 
socket  joint  with  a  long  screw,  the  latter  being  held  in  posi- 
tion by  a  nut.  When  the  current  is  normal,  both  the  nut 
and  screw  revolve  at  the  same  rate,  and  consequently  there 
is  no  axial  movement  of  the  screw,  and  the  brush,  there- 
fore, remains  stationary.  An  electro-magnet,  energized  by 
a  coil  which  is  in  series  with  the  main  circuit,  attracts  an 
armature  whose  movement  toward  the  magnet  is  opposed 
by  the  action  of  a  spring  which  is  susceptible  of  adjust- 
ment. When  the  current  has  too  high  a  value,  the  electro- 


GENERATORS.  205 

magnet  attracts  its  armature  more  strongly  than  ordinarily. 
The  latter  moves  toward  the  magnet,  and  by  its  movement 
catches  a  stop  on  the  revolving  nut,  and  thereby  prevents 
the  revolution  of  the  nut  until  the  resulting  longitudinal 
movement  of  the  screw  has  shifted  the  brushes  sufficiently 


Fig.  125. 

to  bring  the  current  to  its  normal  value.  If  the  current  be 
too  weak,  the  spring  which  is  attached  to  the  electro-magnet 
armature  verpowers  the  magnetic  attraction.  The  result- 
ing movement  of  the  armature  stops  the  rotation  of  the 
screw  and  permits  the  rotation  of  the  nut.  This  results  in 
a  longitudinal  movement  of  the  screw  and  a  shifting  of  the 


206  DYNAMO    ELECTRIC    MACHINERY. 


Fig.  126. 


PROBLEMS.  207 

brushes  in  the  opposite  direction.  The  stopping  and  start- 
ing of  the  nut  and  screw  are  accomplished  through  the 
medium  of  small  triggers  controlled  by  the  armature  of 
the  series  magnet.  The  triggers  are  fastened  to  the  gear 
rotated  from  the  main  shaft  by  a  belt,  and  they  engage  with 
stops  on  the  nut  and  screw  respectively.  Fig.  126  gives 
a  sectional  view  of  the  regulator.  The  trigger  which 
engages  with  the  screw  is  shown  at  nt  and  the  one  which 
engages  with  the  nut  is  shown  at  m. 

PROBLEMS. 

1.  The  resistance  of  the  field  winding  of  a  generator  which 
has  been  standing  idle  for  a  considerable  time  in  an  engine-room 
at  a  temperature  of  25°  C.,  is  22.1   ohms.     The  resistance  of 
this  winding  when  the  generator  is  in  operation  at  full  load  for 
several  hours  is  25  ohms.     Determine  the  temperature  elevation 
of  the  field  coils. 

2.  Estimate  the  output  of  a  generator  the  armature  core  of 
which  is  1 8  inches  in  diameter  and    13   inches  in  length  ;  the 
speed  of  the  machine  being  500  rev.  per  min. 

3.  From  the  following  data  of  a  350-K.W.,  25o-volt,  90  rev. 
per  min.,  i6-pole,  shunt-wound  generator,  determine  the  arma- 
ture core  losses: 

Armature  diameter  108  inches 

Gross  length  of  armature  15  inches 

Net  length  of  armature  1 2  inches 

Number  of  slots  (open  type)  576 

Depth  of  slot  i.o  inch 

Width  of  slot  0.3  inch 
Radial  core  depth  back  of  slots          7  inches 

Conductors  per  slot  2 

Size  of  armature  conductors  0.3  X  0.2  inch 

Flux  per  pole  at  full  load  16  megamaxwells 


208  DYNAMO   ELECTRIC   MACHINERY. 

Type  of  armature  winding  simplex  ;  lap 

Peripheral  length  of  pole  face  15  inches 

Radial  length  of  air-gap  0.3  inch 

Number  of  field  turns  per  pole  600 

Mean  length  of  a  field  turn  59  inches 

Cross-section  of  field  conductor         0.03  sq.  in. 

Drop  at  the  carbon  brushes  2.4  volts 

Current  density  at  brushes  40  amperes  per  3q.  in. 

Diameter  of  commutator  72  inches 

Axial  length  of  commutator  9  inches 

Exposed  surface  per  field  coil  I25o  sq.  in. 

Field  pole  shoes  are  of  laminated  steel. 

4.  What  is  the  armature  copper  loss  at  full  load  of  the  gener- 
ator of  the  foregoing  problem  ? 

5.  Compute  the  pole-face  loss  of   the  350-K.W.  generator, 
the  data  of  which  are  given  under  Prob.  3. 

6.  Calculate  the  excitation  loss  of  the  generator  of  Prob.  3  at 
full  load. 

7.  Find   the  total   commutator   losses    at   full    load   of   the 
350-K.W.  generator  of  Prob.  3. 

8.  Determine  the  temperature   elevations  of   the  armature, 
field  coils,  and  commutator  of  the  generator  discussed  in  the 
foregoing  problems,  when  operating  continuously  at  full  load. 

9.  A  motor-generator  set  consists  of  a  direct-current  generator 
coupled  to  an  alternating-current  synchronous  motor  [power-fac- 
tor =  i].     When  the  generator  delivers  a  current  of  600  amperes 
with  250  volts   across  the  machine  terminals,  the  motor  takes 
26.4  amperes  at  6600  volts.     Determine  the  efficiency  of  the 
motor-generator  set. 

10.  The  E.M.F.  of  a  shunt-wound  railway  generator  rises 
from  550  volts  at  full  load  to  645  volts  upon  disconnecting  the 
load.     What  is  the  percentage  regulation  of  the  machine  ? 

11.  A  shunt-wound  generator,   rated   at  50  K.W.,   supplies 
current  to  an  external  circuit  with  550  volts  across  the  machine 


PROBLEMS.  209 

terminals.  To  produce  this  voltage  6280  ampere-turns  per  pole 
are  required  at  no  load,  and  7640  ampere-turns  at  full  load. 
How  many  series  field  turns  per  pole  must  be  provided  for  flat 
compounding  ? 

12.  A  three-wire  system  supplies  current  to  no-volt  lamps 
and  to  a  220-volt  motor  which  is  connected  to  the  outside  wires. 
It  is  found  that  the  lamps  on  one  side  of  the  system  burn  more 
brightly  than  those  on  the  other,  while  the  motor  operates  as 
usual.     What  is  the  trouble  ? 

13.  Find  the  flux  density  in  the  air-gap  of  the  300-K.W., 
5oo-volt  homopolar  generator  mentioned  in  §  82  ;  the  armature 
diameter  being  taken  as  20  inches  and  its  net  axial  length  as 
12  inches. 

14.  When  a  series-wound  generator  is  driven  at  1200  rev. 
per  min.  its  terminal  voltage  is  150  volts,  with  a  current  output 
of  20  amperes.     Compute  the  terminal  voltage  of  the  machine 
when  it  delivers  a  current  of  50  amperes  at  a  speed  of  1500 
rev.  per  min. ;  the  resistance  of  armature  and  field  windings 
together  being   0.125   onm-     The  increase    in    magnetic   flux 
accompanying  the  increased  current  is  60  %. 


210  DYNAMO   ELECTRIC    MACHINERY. 


CHAPTER    VII. 

MOTORS. 

90.  Principle  of  Action  of  a  Motor.  —  Any  direct-current 
generator  will  operate  as  a  motor  and  deliver  mechanical 
energy  if  supplied  with  current  from  some  external  source. 
This  source  may  be  a  constant-potential  system  or  a  con- 
stant-current system  of  electrical  distribution.  Structur- 
ally generators  and  motors  are  identical,  but  as  motors  are 
generally  placed  as  near  as  possible  to  their  loads,  they  may 
frequently  be  exposed  to  severe  weather  conditions,  dirt, 
etc.,  and  for  this  reason  motors  for  electric  railways,  for 
rolling  mills  and  for  machine  tools  are  of  the  enclosed 
type. 

When  a  current  flows  through  a  conductor  which  is  situ- 
ated in  a  magnetic  field,  a  force  will  be  exerted  upon  that 
conductor  tending  to  move  it  perpendicularly  to  itself  and 
to  the  magnetic  flux ;  the  magnitude  of  this  force  in  dynes 

being  F  =  —,     (§  22) 

where      7  =  current  flowing  in  conductor  in  amperes, 

/=  length  of  conductor  in  centimeters, 
and         (B  =  flux  density  of  magnetic  field  in  gausses. 

Irrespective  of  the  multipolarity  of  the  field  magnets  or  of 
the  method  of  armature  winding,  the  force  actions  between 
the  magnetic  field  and  all  the  currents  in  the  inductors  will 
conspire  to  produce  rotation  in  one  direction. 


MOTORS. 


211 


Consider  a  single  armature  conductor,  Fig.  127,  to  carry 
a  current  flowing  away  from  the  observer.  The  lines  of 
force  which  surround  the  con- 
ductor due  to  the  current  in  it 
will  have  a  clockwise  direction. 
Thus,  to  the  right  of  the  con- 
ductor these  lines  will  have  the 
same  direction  as  the  lines  of 
force  from  the  field  magnet  A7", 
and  to  the  left  of  the  conductor 
they  will  be  opposed  to  the  latter. 
The  resultant  field,  therefore,  will  be  stronger  on  the  right- 
hand  side,  as  shown ;  and  consequently  the  armature  carrying 
that  conductor  will  be  pushed  to  the  left  and  will  rotate 
counter-clockwise. 

91.  Direction  of  Rotation.  —  To  determine  the  direction 
of  movement  of  a  conductor  carrying  a  current  of  definite 
direction  in  a  magnetic  field  of  known  direction,  one  may 


Fig.  127. 


FIELD   MAGNET 

DVNAMcTjFUGHT  HAND. 


HELD  MAGNET 
MOTOR  LEFT  HAND 


Fig.    128. 


Fig.    129. 


employ  a  modification  of  Fleming's  rule.  Thus  in  a  gen- 
erator the  thumb  and  first  two  fingers  of  the  right  hand 
determine  the  direction  of  the  induced  E.M.F.  as  shown 
in  Fig.  128.  But  in  a  motor  the  thumb  and  first  two 


212  DYNAMO   ELECTRIC   MACHINERY. 

fingers  of  the  left   hand  may  determine  the  direction   of 
rotation  as  shown  in  Fig.  129. 

If  in  a  dynamo  the  direction  of  the  field  *  flux  remain 
unaltered,  and  the  armature  be  supplied  with  a  current 
flowing  in  the  same  direction  as  when  the  machine  was 
operated  as  a  generator,  then  the  direction  of  rotation  will 
be  opposite  to  that  while  driven  as  a  generator.  Thus,  if 
the  positive  brush  of  the  generator  be  connected  to  the 
positive  terminal  of  an  external  source  of  supply,  and  if  the 
negative  brush  be  connected  to  the  negative  terminal,  then 
the  direction  of  current  flow  in  the  armature  will  be 
reversed.  The  connections  of  shunt-wound  and  series- 


Fig.  130.  Fig.  131. 

wound  dynamos  are  shown  respectively  in  Figs.  130  and 
131,  in  which  the  full  arrows  represent  generator  condi- 
tions and  the  dotted  arrows  represent  motor  conditions,  the 
connections  to  the  circuit  remaining  unchanged.  In  shunt- 
wound,  separately  excited  and  magneto  machines,  since  the 
magnetic  fields  in  these  dynamos  are  not  reversed,  the 
direction  of  rotation  will  be  unaltered.  The  direction  of 
rotation  of  the  armature  of  series-wound  dynamos,  since 
the  field  flux  also  has  its  direction  changed,  will  be  re- 
versed. Compound-wound  machines  will  have  the  same  or 
reversed  direction  of  rotation,  depending  upon  whether  the 
magnetizing  effect  of  the  shunt  coils  is  stronger  or  weaker 


MOTORS.  213 

than  that  of  the  series  coils.  In  a  compound-wound  gen- 
erator the  actions  of  the  shunt  coils  and  the  series  coils  are 
cumulative,  i.e.,  in  the  same  direction  ;  but  when  used  as  a 
motor  the  actions  are  differential,  i.e.,  opposed  to  each  other. 
Motors  are  also  wound  so  as  to  have  cumulatively  acting 
series  coils. 

To  reverse  the  direction  of  rotation  of  a  motor  one  must 
not  change  the  connections  with  the  supply  mains,  for  this 
would  reverse  the  current  directions  in  both  armature  and 
field  windings,  and  thus  leave  the  direction  of  rotation  unal- 
tered. It  is  necessary  to  change  the  connections  of  either 
field  or  armature  winding,  but  not  of  both. 

92.  Torque  Exerted  by  a  Motor.  —  The  force  which  is 
exerted  upon  each  conductor  carrying  a  current  /  and 
situated  in  a  uniform  magnetic  field  of  flux  density  (B  is 

//(B 
-  dynes,  §  90.    The  total  number  of  conductors  on  the 

armature  which  are  under  the  2p  poles  may  be  represented 
as  kSq, 

where  k  is  the  ratio  of  the  circumferential  length  of  the 
pole  face  to  the  pole  pitch,  5  is  the  number  of  conductors 
in  'series  between  brushes,  and  q  is  the  number  of  current 
paths  through  the  armature  between  brushes.  Let  It  be 
the  total  or  external  armature  current  in  amperes,  and  D 
be  the  diameter  of  the  armature  in  centimeters.  Then  the 
total  torque  exerted  by  the  armature  in  dyne-cm,  is 

T  =  -  •  ^  •  /(B  •  ^  =  0.05 
2      q  10 

But  the  total  flux  per  pole  is 

knDl<$> 


214  DYNAMO   ELECTRIC   MACHINERY. 

Therefore  the  total  torque  in  dyne-cm,  is 


=-.  05 


7T  IO/T 

which  shows  that  the  torque  exerted  by  the  motor  is  pro- 
portional to  the  magnetic  flux  and  to  the  armature  current. 
Since  there  are  980  X  453-6  X  30.48  or  13,549,000  dyne- 
centimeters  in  one  pound-foot,  the  torque  in  pound-feet  may 
be  expressed  as 

T=  2.35  pS&It  io-9. 

The  effective  torque  available  at  the  pulley  of  the  motor 
is  somewhat  less  than  that  given  by  the  foregoing  equation, 
due  to  the  mechanical  and  iron  losses. 

When  load  is  placed  upon  a  motor,  such  as  machinery  in 
one  form  or  another,  a  certain  torque  must  be  exerted  which 
is  equal  to  the  torque-reaction  of  the  load.  With  greater 
load  more  torque  must  be  exerted,  and  therefore  the  prod- 
uct <J>/f  must  become  larger.  As  a  result  a  motor  takes 
more  current  when  operating  under  heavy  load  than  when 
running  light. 

93.  Counter  Electromotive  Force.  —  The  armature  of  a 
motor  revolving  in  a  magnetic  field  under  the  influence  of 
supplied  electrical  energy  differs  in  no  respect  from  the 
same  armature  revolving  in  a  magnetic  field  under  the  in- 
fluence of  supplied  mechanical  energy.  There  is  an  E.M.F. 
generated  in  it  which  is  determined  by  the  speed  and  quan- 
tity of  flux.  For  the  same  speed  and  the  same  flux  there 
would  be  generated  the  same  E.M.F.  in  the  case  of  a  motor 
as  in  the  case  of  a  generator.  The  direction  of  this  E.M.F. 
is,  however,  such  as  to  tend  to  send  a  current  in  a  direction 
opposite  'to  that  of  the  current  flowing  under  the  influence 
of  the  external  supply  of  E.M.F.,  according  to  §  91. 


MOTORS.  215 

Therefore  this  pressure  which  is  induced  in  the  armature 
of  a  motor  is  called  counter  electromotive  force.  The  cur- 
rent which  will  flow  through  the  inductors  of  an  armature 
is  therefore  equal  to  the  difference  between  the  supply 
E.M.F.  and  the  counter  E.M.F.  divided  by  the  resistance 
of  the  armature,  or 

E~EC 
/? 

For  example,  an  unloaded  i-K.W.  shunt-wound  motor 
having  an  armature  resistance  of  I  ohm,  when  connected 
to  a  source  of  constant -potential  supply  of  100  volts,  would 
not  take  a  current  of  100  amperes  as  dictated  by  Ohm's 
law,  unless  its  armature  were  clamped  so  as  to  prevent  ro- 
tation. If  undamped,  its  armature  would  assume  such  a 
speed  that  it  would  have  induced  in  it  a  counter  E.M.F.  of 
say  97.5  volts.  The  current  then  flowing  in  the  armature 
would  be 

100  —  97.5 

-  =  2.5  amperes. 

The  power  represented  by  this  current,  viz.,  2.5  X  100 
watts,  would  all  be  expended  in  overcoming  the  losses  of 
the  machine. 

The  magnitude  of  this  counter  E.M.F.  in  volts  is 

Ec  =  2  p$S —  io~8,  (§  39) 

where  3>  is  the  flux  per  pole  in  maxwells,  and  V  is  the 
speed  in  rev.  per  min. 

If  the  load  upon  a  motor  be  increased,  its  torque  is  no 
longer  sufficient  to  overcome  the  load  and  consequently  its 
speed  drops.  A  lowering  of  speed  implies  the  generation 
of  a  lower  counter  £.M.F.,'a.nd  thus  permits  a  greater 


216 


DYNAMO    ELECTRIC   MACHINERY. 


current  to  flow  through  the  armature.     The  greater  cur- 
rent results  in  a  greater  torque. 

94.  Armature  Reactions.  —  Since  in  a  motor,  for  a  given 
direction  of  rotation  and  of  flux,  the  current  in  the  arma- 
ture flows  in  a  direction  contrary  to  that  which  it  would 

have  as  a  generator,  therefore  the 
effect  of  the  motor  armature  cross 
turns  is  to  distort  the  magnetic  field 
against  the  direction  of  rotation,  as 
in  Fig.  132.  This  increases  the  flux 
density  in  the  leading  pole  tips,  and 
decreases  it  in  the  trailing  tips.  This 
necessitates,  for  sparkless  operation, 
a  backward  lead,  or  a  lag,  of  the 
brushes.  If  the  brushes  were  in  the 
same  place  as  when  the  machine  was 
operated  as  a  generator,  the  direction 
of  armature  current  having  been  re- 
versed, then  the  demagnetizing  or 
back  turns  of  the  generator  would 
become  magnetizing  turns  for  the 
motor ;  but  with  the  brushes  shifted  to  a  position  of  lag, 
then  the  motor  has  also  demagnetizing  or  back  turns. 

95.  Power    of    Motors.  —  The   mechanical  power   of  a 
motor  when  running  at    V  rev.  per  min.  and  exerting  a 
torque   T  dyne-centimeters  is  equal  to  the  product  of  the 
angular  velocity  in  radians  per  second  into  the  torque 


Fig.  132. 


p  = 


60 


T. 


But  the  torque  exerted,  in  dyne-centimeters,  is 

(§92) 


I07T 


MOTORS.  217 

It 


=  [2  p<$>S 
I  l 


Therefore  P  =  ,  -  „  .  ~  £ 

60 J  io 

But  the  quantity  in  the  brackets  is  equal  to  ios  Ec.  (§93) 

Whence 

dyne-cm. 

P  =  EJt  io7  — =  EJt  watts. 

sec. 

Thus,  the  rate  at  which  a  motor  does  mechanical  work  is 
equal  to  the  product  of  the  counter  electromotive  force  gen- 
erated, in  volts,  into  the  total  current  flowing  through  the 
armature  in  amperes. 

Shunt  Motors. 

96.  Speed  of  Shunt  Motors.  —  In  shunt-wound  motors 
connected  to  constant-potential  supply  circuits  the  field 
current  is  constant  and  consequently  the  magnetic  field  is 
of  unvarying  intensity.  Solving  the  equations  of  §  93  for 
speed,,  there  results 

(E  —InR^t  60  .  i  o8 


and  therefore  if  <3>  is  constant  the  speed  of  the  motor  will 
be  practically  constant.  It  will  not  be  absolutely  constant 
because  the  small  resistance  drop  occasions  a  slight  lower- 
ing of  speed  with  increased  load  on  the  machine.  On  the 
other  hand  the  effect  of  the  armature  current  is  to  weaken 
the  magnetic  field,  if  the  brushes  be  displaced  backward 
from  the  neutral  plane,  and  thereby  tend  to  increase  the 
speed.  This  partially  counteracts  the  lowering  of  speed 
due  to  resistance  drop.  The  speed  variation  of  shunt 
motors  from  no  load  to  full  load  ranges  from  2  to  io  per 
cent  of  the  speed  at  no  load,  the  lower  value  representing 
that  for  large  machines. 


218 


DYNAMO   ELECTRIC    MACHINERY. 


An  inspection  of  the  foregoing  equation  suggests  the  fol- 
lowing possible  ways  of  controlling  the  speed  of  a  motor: 
(i)  changing  the  exciting  current  in  order  to  change  the 
magnetic  flux  passing  through  the  armature,  (2)  changing 
the  resistance  of  the  armature  circuit,  and  (3)  changing 
the  impressed  electromotive  force.  A  slight  change  of 
speed  can  be  effected  by  shifting  the  position  of  the 
brushes,  for  at  a  given  load  the  speed  is  a  minimum  with 
the  brushes  in  the  neutral  plane,  and  it  will  be  increased 
by  a  lag  of  the  brushes  ;  commutation  difficulties  limit  the 
speed  variation  by  this  method. 

(i)  A  rheostat  placed  in  the  field  circuit  of  a  shunt 
motor  may  be  used  to  vary  its  speed,  Fig.  133.  By 

increasing  the  amount  of 
resistance  in  this  rheo- 
stat the  current  in  the 
field  coils  will  be  de- 
creased; this  results  in 
a  weaker  magnetic  field, 
and  consequently  the 
motor  will  run  at  a 
higher  speed.  If  the 
iron  of  the  magnetic  cir- 
cuit is  well  saturated,  a  considerable  change  in  resistance 
is  necessary  materially  to  alter  the  field  intensity.  A  large 
exciting  current  is  then  required  to  increase  the  magnetic 
flux,  and  this  may  occasion  excessive  heating  of  the  field 
coils.  Again,  if  an  attempt  be  made  to  reduce  the  flux  to 
a  considerable  extent  by  introducing  more  resistance  to 
obtain  a  high  speed,  the  demagnetizing  effect  of  the  arma- 
ture current  will  be  greater  upon  the  weakened  magnetic 
field,  and  consequently  serious  sparking  will  result.  Thus 


Shunt  Field 


Fig.  133- 


MOTORS. 


219 


armature  reaction  limits  speed  variation.  A  shunt  motor 
of  the  usual  type  which  operates  at  a  speed  of  V  rev.  per 
min.  when  the  iron  of  its  magnetic  circuit  is  near  saturation 
will  operate  satisfactorily  at  any  speed  up  to  say  2  V  revolu- 
tions per  minute.  Field  rheostats  are  described  in  §  77. 

In  order  to  vary  the  speed  of  a  shunt  motor  over  a  wide 
range  by  this  method  it  is  necessary  to  neutralize  the  effect 
of  armature  reaction.  This  neutralization  is  accomplished 
by  the  provision  of  a  reversing  magnetic  field  obtained  by 
the  insertion  of  auxiliary  poles,  called  commutating-poles 
or  inter-poles,  between  the  field-magnet  poles.  The  coils 
on  these  auxiliary  poles  are  connected  in  series  with  the 
armature,  as  shown  in  Fig.  134,  and  therefore  the  magnetic 


i 


Fig.  134- 

flux  from  them  is  practically  proportional  to  the  armature 
current.  The  reactance  voltage  (§  57)  generated  in  a  short- 
circuited  armature  coil  due  to  its  rotation  in  the  main  mag- 
netic field  is  also  proportional  to  the  current  flowing  in  it. 
The  M.M.F.  of  the  interpoles  is  adjusted  so  that  a  magnetic 
field  is  produced  in  the  commutatingzone  of  such  magnitude 
that  an  E.M.F.  is  generated  in  the  short-circuited  coils  by 
their  rotation  which  is  equal  but  opposite  to  the  reactance 
voltage.  The  action  of  the  interpoles  is  therefore  entirely 


220  DYNAMO   ELECTRIC   MACHINERY. 

automatic  and  enables  sparkless  commutation  at  all  loads 
and  speeds.  Interpole  motors  are  particularly  adapted  for 
individual  motor  drive  of  machine  tools  and  for  elevator 
operation,  where  large  speed  variations  are  essential.  The 
Electro-Dynamic  Company  manufactures  such  motors,  Fig. 


Fig.  135. 

135,  which  operate  at  a  speed  of  from  100%  to  600%  of 
the  minimum  speed. 

The  data  of  a  5-H.P.  interpole  motor  follow  : 
Resistance  of  shunt  field  175  ohms, 

Resistance  of  armature  1.18  ohms, 

Resistance  of  interpolar  windings  0.2 1  ohm, 
Armature  current  at  full  load       22  to  24  amperes, 
Field  current  0.15  to  1.26  amperes, 

Speed  200  to  1 200  rev.  per  min., 

Weight  1 200  pounds. 


MOTORS. 


221 


A  change  in  magnetic  flux  can  also  be  accomplished  by 
varying  the  reluctance  of  the  magnetic  circuit,  the  field 
current  remaining  unaltered.  The  reluctance  may  be  in- 
creased by  lengthening  the  air-gap  of  the  motor ;  this 
decreases  the  flux  and  consequently  produces  a  higher 
speed.  A  variable-speed  motor  depending  upon  change 
of  reluctance  for  speed  control  is  shown  in  Fig.  136, 


Fig.  136. 

which  depicts  a  4-pole  machine  of  the  Stow  Manufac- 
turing Company.  The  field  cores  are  hollow  and  are 
provided  with  movable  iron  poles,  the  positions  of  which 
are  simultaneously  shifted  by  means  of  hand  wheel  and 
gears.  Large  air-gaps  are  conducive  to  sparkless  com- 
mutation. 

(2)  The  speed  of  a  shunt-wound  motor   with  constant 
excitation  may  be  varied  by  introducing  a  variable  resistance 


222  DYNAMO   ELECTRIC   MACHINERY. 

in  the  armature  circuit.  The  use  of  this  method  of  speed 
control  is  not  to  be  advised  save  for  experimental  purposes, 
since  it  is  very  wasteful  of  energy.  The  PR  loss  in  the 
regulating  resistance  at  certain  speeds  is  considerably  more 
than  the  power  required  by  the  motor.  Further,  the  speed, 
when  reduced  in  this  way,  changes  very  considerably  when 
the  load  on  the  motor  is  altered. 

(3)  Changing  the  electromotive  force  impressed  upon 
the  armature  of  a  shunt  motor  will  cause  a  corresponding 
change  in  speed.  Speed  control  by  this  method  may  be 
accomplished  by  subdividing  the  generator  voltage  into 
two  or  more  components,  and  by  supplying  current  at  these 
different  voltages  over  a  number  of  line  wires  to  the  motor. 
A  controller  is  provided  by  means  of  which  the  motor 
armature  may  be  connected  to  any  pair  of  supply  mains, 
the  field  winding  of  the  machine  being  always  connected 
to  a  definite  pair  of  them.  The  main  generator  voltage 
is  subdivided  by  a  set  of  generators,  called  a  balancer ; 
Fig.  181  illustrates  a  three-element  balancer  for  a  4-wire 
multivoltage  distribution  system.  The  connections  of  a 
motor  to  such  a  system  through  a  controller  are  shown  in 
Fig.  137.  Six  different  voltages  are  obtainable,  namely, 
40,  80,  1 20,  1 60,  200  and  240  volts,  by  moving  the  con- 
troller handle.  The  motor  speeds  under  these  voltages  are 
approximately  proportional  to  the  voltages  themselves,  so 
that  this  method  of  speed  control  gives  a  number  of  defi- 
nite and  widely  different  speeds.  Intermediate  speeds  may 
be  obtained  by  weakening  the  magnetic  fields  of  the  motors 
using  field  rheostats  as  described.  Controllers  designed 
to  perform  both  of  these  functions  are  also  employed.  This 
system  is  extensively  used  in  machine  shops  for  driving 
lathes,  planers,  and  similar  machines. 


MOTORS. 


223 


Speed  control  of  shunt  motors  by  varying  the  voltage 
impressed  upon  the  armature  is  also  the  principle  of  the 
Ward  Leonard  system,  which  differs  from  the  foregoing 
multivoltage  system  in  that  a  finely  graded  variation  of 
speed  is  made  possible  by  the  employment  of  a  separate 
generator  which  supplies  current  to  the  motor.  This  gen- 
erator, G,  Fig.  138,  is  driven  at  constant  speed  by  any  type 


CONTROLLER 


Fig.  137- 


of  prime  mover,  S,  or  by  an  auxiliary  shunt  motor  which 
takes  current  from  the  supply  mains.  An  adjustable  resist- 
ance in  the  generator  field  circuit  regulates  the  voltage 
which  is  impressed  upon  the  motor  armature,  M,  from  prac- 
tically zero  to  its  maximum  value.  The  field  winding  of 
the  motor  is  connected  directly  to  the  supply  circuit,  so 
that  the  intensity  of  its  magnetic  field  is  constant. 

When  it  is  desired  to  start  the  motor,  the  rheostat  is  ad- 
justed so  that  a  high  resistance  is  in  circuit  with  the  field 


224 


DYNAMO   ELECTRIC   MACHINERY. 


winding  of  the  generator;  thus  current  at  low  voltage  will 
be  supplied  to  the  motor.  The  latter  then  starts  to  revolve 
slowly.  To  accelerate  the  motor,  more  resistance  is  cut 
out  of  the  generator  field  circuit  and  consequently  the  volt- 
age across  the  motor  armature  terminals  increases,  thus 
resulting  in  higher  speed. 


Fig.  138. 

This  system  of  motor  control  is  especially  advocated  for 
operating  guns  and  turrets  on  battleships,  where  thorough 
control  is  essential.  For  the  latter,  the  field  rheostats  are 
designed  to  yield  seventy  or  more  speeds,  the  maximum 
speed  being  usually  100  degrees  per  minute.  The  turret- 
turning  motors  are  rated  at  25  and  15  H.P.  respectively 
for  12-inch  and  8-inch  turrets.  The  gun-elevating  motors 
are  rated  at  8  and  5  H.P.  for  1 2-inch  and  8-inch  guns 
respectively. 

Of  the  various  methods  of  speed  control  just  described, 
the  field  rheostat  method  is  perhaps  the  most  used.  It  is 
simple,  cheap,  and  enables  the  speed  to  be  kept  at  definite 
value  under  changes  of  load.  Its  range  is  limited  in  shunt 
motors  of  the  usual  type,  but  the  interpole  motor  removes 


MOTORS. 


225 


this  difficulty.  The  change  of  reluctance  method  does  not 
require  a  field  rheostat,  but  this  is  offset  by  the  increased 
cost  of  construction  of  such  motors.  Both  the  multivolt- 
age  and  Ward  Leonard  systems  are  very  practical  but  ex- 
pensive, since  the  former  requires  a  balancer,  a  number 
of  live  wires,  and  individual  controllers  ;  and  the  latter  sys- 
tem requires  a  motor-generator  set  and  rheostat  for  each 
motor. 

97.  Starting  of  Shunt  Motors.  When  the  armature  of 
a  motor  is  at  rest  there  is  no  counter  E.M.F.;  and  at  the 
instant  of  closing  the  circuit  a  destructive  current  would 
flow  if  a  resistance  were  hot  first  inserted  in  the  circuit, 
except  in  the  case  of  very  small  motors  whose  armatures 
have  small  moments  of  inertia.  As  the  speed  rises  the 
counter  electromotive  force  in- 
creases and  the  current  is  reduced, 
thus  permitting  the  resistance  to 
be  gradually  lessened  without  caus- 
ing an  excessive  current  to  flow 
through  the  armature.  When  the 
speed  approaches  its  ultimate  value 
this  resistance  is  entirely  cut  out 
of  circuit.  In  order  that  the  counter 
E.M.F.  may  be  generated  the  shunt 
field  circuit  must  be  closed,  so  that 
the  armature  conductors  cut  lines 
of  force.  An  arrangement  for  con- 
veniently performing  these  func- 
tions is  called  a  starting  box  or 
starting  rheostat. 

The  connections  of  a  simple  starting  rheostat  are  shown 
in  Fig.  1 39.     Its  main  feature  is  a  contact  arm  capable  of 


Shunt 


Fig-   139- 


226 


DYNAMO   ELECTRIC    MACHINERY. 


rotation  about  its  center  so  that  one  end  moves  over  a 
series  of  contact  studs  while  the  other  makes  contact  with 
the  segment  which  is  connected  to  the  field  winding.  As 
the  arm  is  slowly  turned  around,  it  first  completes  the  field 
circuit,  then  the  other  end  touches  the  first  contact  stud, 
thereby  closing  the  armature  circuit  through  all  the  resist- 
ance of  the  rheostat.  As  the  speed  increases  the  revolv- 
ing arm  cuts  out  more  and  more  of  the  resistance,  until 
finally  the  armature  is  operating  on  the  full  voltage  of  the 
supply  circuit. 


Release  Magnet 


Fig.   140. 


A  shunt  motor  may  have  its  armature  coils  destroyed 
by  an  excessive  rush  of  current  resulting  from  a  dropping 
or  interruption  of  the  supply  voltage  followed  by  a  sudden 
renewal  after  the  speed  of  the  armature  has  fallen.  These 
conditions  may  arise  through  accidents  to  supply  mains  or 
because  of  an  extremely  heavy  load  on  mains  of  insufn- 


MOTORS.  227 

cient  cross-section.  An  armature  may  also  be  burned  out 
by  an  excessive  current  due  to  overloading  the  motor.  The 
resulting  lowering  of  its  speed  is  accompanied  by  a  corre- 
sponding lowering  of  the  counter  E.M.F.  Again,  an  abnor- 
mal voltage,  which  might  result  from  some  cross  or  other 
accident,  might  cause  a  destructive  rush  of  current.  To 
meet  these  conditions,  starting  rheostats  are  often  pro- 
vided with  attachments  for  opening  the  circuit  on  no 
voltage  or  low  voltage, 
and  others  with  attach- 
ments for  opening  the 
circuit  on  overload. 
Some  have  both  attach- 
ments, but  it  is  modern 
practice  to  place  the 
overload  device  upon 
the  switchboard  rather 
than  on  the  starting 
rheostat.  Fig.  140  is 
a  wiring  diagram  of  a 
shunt-motor  starting 
b'ox  with  automatic  re- 
lease and  no-voltage 
attachment. 

A  view  of  a  motor- 
starting  panel  with  both 
no-voltage  and  overload 
attachments  is  given  in 
Fig.  141.  When  the  Fig'  '4'' 

handle  is  placed  in  the  "  on  "  position,  the  magnet  in  the 
field  circuit  holds  it  there,  although  a  spring  tends  to 
throw  it  back.  If  now,  because  of  low  voltage,  the  cur- 


228 


DYNAMO   ELECTRIC   MACHINERY. 


rent  in  field  winding  and  magnet  becomes  low,  the  magnet 
is  no  longer  able  to  retain  the  handle,  and  the  spring  throws 
it  to  the  "  off  "  position,  where  it  stays  until  the  motor  is 
again  turned  on  by  an  attendant.  The  overload  coil  is 
connected  in  series  with  the  motor  armature,  and  on  over- 
load becomes  strong  enough  to  attract  an  iron  piece.  This 
operation  places  a  short-circuit  on  the  release  magnet, 
which  therefore  permits  the  starting  arm  to  spring  back  to 
the  "off"  position.  This  panel  is  provided  with  a  main 
switch  and  enclosed  fuses,  although  the  latter  are 

frequently  replaced  by 
circuit  breakers.  Self- 
contained  motor-starting 
panels  avoid  consider- 
able external  wiring, 
thereby  increasing  relia- 
bility, are  quickly  in- 
stalled, and  add  to  the 
neatness  of  the  equip- 
ment. 

A  combined  starting 
and  field-regulating  rhe- 
ostat made  by  the  Cutler- 
Hammer  Manufacturing 
Company  is  shown  in 
Fig.  142.  This  type  of 
apparatus  is  designed  for 
2  to  i  up  to  5  to  i  speed 
variation.  The  movable 
arm  consists  of  two  parts 
which  wipe  over  separate  sets  of  contacts.  To  start  the 
motor  the  handle  is  moved  to  the  extreme  right,  in  which 


Fig.  142. 


MOTORS. 


229 


position  the  magnet  will   hold  the  lower  portion   of  the 
arm.     The    upper   arm    is    then  free  to   move   back  and 


Fig.  143- 

make  contact  with  the  studs  joined  to  the  field-regulating 
resistance. 

Fig.  143  depicts  a  General  Electric 
Company  controller  for  5-H.P.  shop-tool 
motors.  There  are  three  starting  points, 
21  forward  and  1 1  reverse  running  points. 
Speed  control  is  effected  by  field  regu- 
lation. 

A  self-starter  for  shunt  motors  made 
by  the  Ward  Leonard  Electric  Company 
is  shown  in  Fig.  144.  It  consists  of 
an  electromagnet  with  a  movable  core 
carrying  wipers  which  make  contact  with 
a  series  of  studs  as  the  core  is  attracted  Flg' I44< 

by  the  magnet.     The  rapidity  with  which  this  operation 


230 


DYNAMO   ELECTRIC   MACHINERY. 


may  be  performed  is  controlled  by  a  dash-pot.  Thus  the 
starting  and  stopping  of  the  motor  is  accomplished  simply 
and  effectively  by  means  of  a  main  line  switch. 

98.  Design  of  Starting  Rheostats.  —  The  design  of  a 
starting-box  for  a  shunt  motor  under  constant  excitation  is 
governed  by  the  permissible  starting  current  through  the 
motor  armature.  This  maximum  current  value  is  usually 
specified  in  terms  of  the  full-load  current  of  the  machine. 
Let  f  be  the  ratio  of  maximum  starting  current  under  load 
to  the  full-load  current  ;  this  ratio  is  always  greater  than 
unity.  Let  rlt  r2,  ra,  .  .  .  ,  r  be  the  resistances  respectively 
of  the  starting-box  when  the  rheostat  arm  is  on  contact 


Fig-  145- 

studs  i,  2,  3,  ...,»,  Fig.  145.  At  the  instant  when  the 
arm  touches  stud  i,  the  current  flowing  through  the  arma- 
ture is 

E 


where  E  is  the  line  voltage  and  Ra  is  the  resistance  of  the 
motor  armature.     When  the  motor  runs  at  constant  speed 


MOTORS.  231 

with  this  rheostat  setting,  the  current  flowing  through  the 
armature  is 

F       F 

\    7  =  ^f>  <2) 

where  ECI  is  the  counter  E.M.F.  generated  at  this  speed. 
The  rheostat  arm  may  then  be  turned  to  contact  stud  2, 
and  this  results  in  a  momentary  increase  of  current, 


This  causes  the  motor  to  exert  a  greater  torque  than  that 
necessary  to  overcome  the  load  and  consequently  the  motor 
is  accelerated  and  will  assume  some  higher  speed.  The 
current  will  then  diminish  to 


where  £C2  is  the  E.M.F.  generated  at  the  increased  speed. 
Similarly,  when  the  arm  makes  contact  with  stud  3,  the 
current  flowing  through  the  motor  armature  will  again 
increase  to 


From  equations  (2)  and  (3)  and  equations   (4)  and  (5) 
there  result  respectively 


Ra 
There  are  n  such  equations,  the  last  one  being 


The  number  of  steps  into  which  the  total  resistance  ^  is  to 
be  divided,  so  that  the  starting  current  shall  not  exceed  the 


232  DYNAMO   ELECTRIC    MACHINERY. 

specified  value  of  7-7  amperes,  may  be  determined  from  the 
product  of  these  n  equations,  which  is 


But  from  (i) 
consequently 


R 


logr 

If  the  motor  is  to  be  accelerated  from  rest  to  full  speed 
without  any  load  on  it,  fewer  steps  are  required,  because 
the  no-load  current  is  much  less  than  the  full-load  current. 

The  resistance  of  each  of  the  various  steps  may  then  be 
computed  ;  thus  for  the  first  portion  between  studs  I  and 
2  the  resistance  is,  from  (6), 


and  similarly  the  resistance  of  the  next  part  is 
r2  -r3=  (Ra  +  r2)  (i-  -V  etc. 

The  resistances  of  the  various  steps  of  a  starting  rheostat 
will  be  found  in  examples  to  differ  from  one  another.  Some- 
times additional  steps  are  provided,  so  that  the  maximum 
permissible  current  will  not  flow  through  the  armature 
when  the  rheostat  arm  touches  the  first  stud,  but  only 
when  contact  is  made  with  the  second  or  third  stud. 

99.    Speed   Regulation.  —  A  shunt-wound   motor  under 
constant  impressed  terminal  voltage  will  have  an  approxi- 


MOTORS.  233 

mately  constant  speed.  It  will  decrease  somewhat  as  the 
load  on  the  machine  increases.  The  principal  cause  of  this 
speed  variation  under  varying  load  is  the  change  of  the 
armature  resistance  drop,  and  it  is  therefore  desirable  that 
the  resistance  of  the  armature  be  small.  The  change  of 
speed,  with  a  fixed  setting  of  the  field  rheostat,  from  full 
load  to  no  load,  expressed  in  terms  of  the  speed  at  full  load, 
is  called  the  speed  regulation  of  the  motor.  For  example, 
the  speeds  of  a  shunt  motor  at  no  load  and  at  full  load  are 
860  and  825  rev.  per  min.  respectively.  Consequently  its 
speed  regulation  is 

860  -  825 

-— =  .0425,  or  4j  per  cent. 

025 

The  maintenance  of  a  strictly  constant  speed  necessitates 
the  manipulation  of  a  field  rheostat,  that  is,  the  adjustment 
of  a  device  external  to  the  motor  itself.  Speed  regulation 
is  to  be  distinguished  from  speed  control.  The  former  in- 
dicates the  speed  changes  inherent  in  the  machine,  whereas 
the  latter  means  adjustment  for  various  desired  speeds. 

100.  Characteristic  Curves  of  Shunt  Motors.  —  The  char- 
acteristic curves  of  a  motor  include  curves  of  speed,  effi- 
ciency, current  input,  and  torque,  in  terms  of  the  H.P. 
output  of  the  machine.  Such  curves  for  a  /.5-H.P.,  230 
volt  General  Electric  Company  Type  CQ  motor  are  shown 
in  Fig.  146. 

A  shunt  motor  when  started  cold  on  no  load  quickly 
arrives  at  a  speed  which  then  gradually  rises  to  a  maximum. 
The  gradual  heating  of  the  field  coils  increases  their  resist- 
ance. This  allows  less  current  to  flow  in  them,  and  the 
resulting  magnetic  flux  is  less.  Therefore  the  armature 
must  rotate  faster  to  generate  the  same  counter  E.M.F. 


234 


DYNAMO    ELECTRIC   MACHINERY. 


The  efficiency  of  a  motor  is  the  ratio  of  the  mechanical 
output  to  the  electrical  input.  The  determination  of  the 
output  may  be  made  directly  by  experiment,  or  it  may  be 


120 


1,0 


t 


50      1000 


30£    GOO 


H.  P.  OUTPUT 

Fig.  146. 

found  from  the  measurement  of  the  losses.  The  latter 
method  is  to  be  preferred  because  of  its  greater  accuracy. 
The  various  losses  may  be  obtained  as  in  Chap.  VI.  Thus, 
the  efficiency  is 

746  H.P. 


746  H.P.+(Ph  +  Pe  +  Pa  +  Pp+Pf+Pfw+Pc)' 


(§69) 


The  efficiency  at  any  load  should  be  determined  at  the  ulti- 
mate temperature  assumed  under  continuous  operation  at 


.  MOTORS. 


235 


that  load,  referred  to  the  standard  engine-room  temperature 
of  25°  C. 

In  the  direct  determination  of  motor  output,  any  con- 
venient load  may  be  placed  on  the  machine.  For  the 
smaller  motors  a  Prony  brake  may  be  used,  the  strap-brake 
being  a  convenient  form,  Fig.  147.  The  power  absorbed 
in  watts  is  expressed  as 


Output  = 


33000 


where  r  is  the  radius  of  the  pulley  in  feet,  V  is  the  speed 
in  rev.  per  min.,  and  (P  —Pf)  is  the  difference  of  the  two 
scale  readings  \\\  pounds. 

For  large  motors  the  load  usually  consists 
of  generators,  the  output  of  which  may  be 
absorbed  by  suitable  resistances.  If  the 
generator  be  of  proper  voltage  the  current 
therefrom  may  be  returned  to  the  sup- 
ply circuit.  This  method  of  loading  a 
motor,  called  the  loading-back  method, 
results  in  a  considerable  saving  of  power, 
since  the  net  power  taken  from  the  mains 
is  only  that  required  to  supply  the  losses 
of  the  two  machines.  The  amount  of  Fi2'  I47> 
load  is  regulated  by  changing  the  field  current  of  the 
generator. 

The  efficiency  of  a  shunt  motor  at  full  load  can  be  esti- 
mated from  the  data  stamped  on  the  name  plate  of  the 
machine.  Thus,  for  the  following  data  : 


H.P.  — 20 
Volts —  1 20 


Amp.  —  150 
R.P.M.  —  925 


236  DYNAMO   ELECTRIC   MACHINERY. 

the  efficiency  at  full  load  is 

e=    746  X  20   =          Qr 

120  X    ISO 

10 1.    Industrial    Applications    of    Shunt    Motors. — The 

design  of  motors  differs  frequently  in  many  details  from 
that  of  generators,  especially  if  the  motors  are  to  be  di- 
rectly coupled  to  the  machines  they  drive.  These  points 
of  difference  are  principally  in  the  construction  of  the 

frame,  bearing  supports 
and  shafts.  Often  motors 
must  be  placed  out  of 
doors  or  where  they  are 
exposed  to  dust,  chips, 
etc.;  in  such  cases  they 
should  be  of  the  enclosed 
type. 

Fig.  148  depicts  a  back- 
geared  motor  driving  a 
Hamilton  drill  press.  This 
type  of  motor  is  desirable 
for  slowly  moving  ma- 
chines, since  it  permits  of 
the  usual  high  armature 
speeds.  A  protecting 
guard  surrounding  the 
gear  and  pinion  is  usu- 
ally furnished  to  prevent 

Fig.  148.  \  1 

accidents. 

A  Crocker -Wheeler  adjustable-speed  motor  directly 
geared  to  a  36-inch  lathe  is  shown  in  Fig.  149.  Motors 
so  situated  are  usually  of  the  open  type,  but  they  are 


MOTORS. 


237 


Fig.  149- 


Fig.  150. 


238 


DYNAMO   ELECTRIC   MACHINERY. 


sometimes  provided  with  gridiron  covers  and  gauze  to  give 
better  protection  against  dirt. 

The  costs  per  hour  of  operating  machine  tools  driven  by 
individual  motors  are  given  in  the  following  table,  the  data 
representing  conditions  such  as  obtain  in  large  machine 
shops.  Fixed  charges  include  interest  and  insurance  on 
investment  in  buildings  and  equipment,  variable  charges 
include  repairs  and  renewals,  and  salaries  include  cost  of 
management,  engineering,  labor,  etc.;  these  charges  are 
apportioned  among  the  various  machines. 


HOURLY  OPERATING  EXPENSE  IN  DOLLARS. 

c/5 
H 

H 

8 

2 

O 

K 

M 

TYPE  OF 
MACHINE. 

SIZE. 

i 

u 

if 

a 

°8 

H  J 

s 

TOTAL. 

H 

Q 

H 

Z 

H 

VERTICAL 
BORING 
MILLS 

4o"-6o" 

72//-IOO" 

10'  -14' 
16'  -24' 

.02 

.04 

.05 
.08 

•  25 

•45 
.80 

2.00 

15 

-25 
.40 
i  .00 

•05 
.08 

•15 
•30 

'08 
•30 

.01 
.OI 
.02 

0-53 
O.9I 

1-57 

RADIAL 

5' 

.02 

•3° 

.20 

•03 

•03 

.01 

0-59 

DRILLS 

10' 

.04 

.60 

•35 

.09 

.09 

.01 

1.18 

ENGINE 

3o"-4o" 

.02 

•25 

.  12 

.04 

.04 

.01 

0.48 

LATHES 

40  "-60" 

•03 

50 

•25 

.  10 

.10 

.01 

0.99 

36"-$6" 

.04 

•55 

•30 

.05 

-OS 

.01 

i  .00 

PLANERS 

1'  -10' 

.06 

I  .10 

.60 

•T5 

.02 

2.08 

12'  -14' 

•T5 

2.60 

I  .40 

•25 

•25 

•03 

4.68 

Power  for  machine  tool  operation  may  be  furnished 
either  by  individual  motors  or  from  a  line  shaft.  The  ini- 
tial investment  for  line-shaft  drive  is  usually  less  than  for 
individual  motor  drive,  but  the  latter  is  conducive  to  in- 
creased production.  Heavier  cuts  are  possible  and  the  time 
for  a  given  operation  is  shorter  with  individual  motors. 

Fig.  150  illustrates  the  operating  mechanism  of  the  Otis 


MOTORS.  239 

Traction  Elevator,  which  consists  essentially  of  a  slow- 
speed  shunt-wound  motor,  a  sheave,  and  a  brake  pulley,  the 
latter  enveloped  by  a  pair  of  powerful  spring-actuated  and 
electrically  released  brake  shoes,  all  compactly  grouped 
on  a  heavy  iron  bedplate.  The  armature  shaft  serves  as  a 
support  for  the  elevator  car  and  counterweight,  and  on 
it  are  mounted  the  sheave  and  brake  pulley,  the  drive 
between  the  armature  spider  and  sheave  being  effected 
through  the  engagement  of  projecting  arms  on  each  cush- 
ioned by  rubber  buffers.  A  controller  is  used  for  accel- 
erating and  retarding  the  car.  The  control  equipment  is 
so  designed  that  the  cars  are  automatically  and  gradually 
retarded  and  brought  to  rest  at  the  upper  and  lower  termi- 
nals of  travel,  an  operation  which  is  entirely  independent 
of  the  position  of  the  car  controller.  Apparatus  of  this 
kind  is  installed  in  the  Singer  and  Metropolitan  towers  in 
New  York  City,  and  enables  one  to  reach  the  fortieth 
floors  of  these  buildings  from  the  street  level  in  less  than 
one  minute. 

Seri:s  Motors. 

1 02.  Series  Motors.  —  As  the  current  traversing  the  field 
windings  of  a  series-wound  motor  is  the  same  as  that  which 
flows  through  its  armature,  the  field  strength  of  such  a 
machine  will  vary  with  the  load  placed  upon  it.  Torque, 
being  proportional  to  the  product  of  the  magnetic  flux  and 
the  armature  current,  §  92,  will  vary  approximately  as  the 
square  of  the  current  taken  by  the  motor.  This  is  true  for 
a  series  motor  with  an  un saturated  magnetic  circuit,  but  in 
practice  the  magnetic  circuit  is  designed  to  approach  satu- 
ration near  the  rated  output,  and  consequently  the  torque 
exerted  varies  to  a  smaller  extent  than  the  square  of  the 
current. 


240  DYNAMO   ELECTRIC   MACHINERY. 

The  speed  of  a  series  motor  in  revolutions  per  minute  is 

(E  -  IR)  60 
F=v  -          -^ — io8,  (§93) 

2p$S 

where  E  is  the  impressed  E.M.F.,  R  is  the  combined  re- 
sistance of  armature  and  field  windings,  /  is  the  number  of 
pairs  of  poles,  3>  is  the  total  flux  per  pole,  and  5  is  the 
number  of  armature  conductors  in  series  between  brushes. 
An  inspection  of  this  expression  shows  that  with  increased 
load  the  numerator  will  be  but  slightly  altered  because  R 
is  small,  and  that  the  denominator  will  be  considerably 
increased  since  <l>  varies  with  the  current.  Consequently 
the  speed  of  the  motor  decreases  as  the  load  increases. 

The  speed  of  the  armature  of  a  series  motor  will  be  such 
that  the  counter  E.M.F.  generated  at  that  speed  will  re- 
duce the  current  to  a  proper  value,  so  that  the  total  power 
consumed  will  be  equal  to  the  sum  of  the  motor  output 
and  the  losses.  In  a  shunt-wound  motor,  a  very  small 
variation  of  speed  is  sufficient  to  compensate  for  a  wide 
variation  of  load.  With  decrease  of  load  both  shunt  and 
series  motors  speed  up  and  generate  a  higher  counter 
electromotive  force.  The  resulting  decrease  of  current 
causes,  in  the  series  machine,  a  weakening  of  the  magnetic 
field,  and  as  a  consequence  additional  speed  is  required  to 
maintain  this  E.M.F.  Thus  a  small  variation  in  load  on  a 
series  machine  results  in  a  wide  change  of  speed. 

The  exertion  of  a  large  torque  at  low  speeds  and  a  small 
torque  at  high  speeds  results  in  a  rather  uniform  energy 
consumption,  for  power  output  equals  the  product  of  torque 
and  angular  velocity.  For  this  reason  the  series  motor  is 
particularly  suitable  for  traction  and  for  the  operation  of 
cranes  and  of  rolling  mills.  A  series-wound  machine  can 


MOTORS.  241 

be  used  on  either  a  constant-current  circuit  or  on  a  con- 
stant-potential circuit ;  but  a  series  motor  is  seldom  run  on 
a  constant-potential  circuit  unless  it  is  directly  or  very 
solidly  coupled  with  its  load.  If  connected  by  means  of  a 
belt,  and  if  the  belt  should  break  or  slip  off,  the  motor 
would  speed  up  indefinitely  and  cause  the  armature  to  fly 
to  pieces.  The  series  motor,  therefore,  cannot  be  run  at 
no  load  and  rated  voltage.  This  difficulty  does  not  pre- 
sent itself  when  series  motors  are  operated  on  constant- 
current  circuits,  a  practice  no  longer  in  vogue. 

103.  Characteristic  Curves  of  Series  Motors.  — The  char- 
acteristic curves  of  a  5-H.P.,  22O-volt,  back-geared,  series- 
wound  motor  are  shown  in  Fig.  151,  and  include  curves  of 
current  input,  torque,  speed  and  efficiency,  plotted  in  terms 
of  the  horse-power  output.     The  high  speeds  attained  when 
the  motor  is  under  light   loads  are  clearly  indicated   by 
the  speed-output  curve.     Frequently  curves  of   torque  in 
terms  of  speed  are  used,  especially  in  the  selection  of  motor 
capacity  for  electric  cars  or  locomotives.      Fig.  152  depicts 
such  a  curve  for  the  5-H.P.  motor  mentioned  above. 

If  a  series  motor  be  at  rest  and  the  circuit  be  closed,  an 
enormous  rush  of  current  will  occur,  giving  a  tremendous 
torque.  Destructive  heating  and  sparking  would  probably 
result.  To  prevent  damage  it  is  therefore  necessary,  in 
the  operation  of  these  motors,  to  insert  at  the  start  a  series 
resistance  which  may  be  cut  out  after  the  speed  has  risen 
enough  to  give  a  sufficient  counter  E.M.F.  In  practice 
controllers  are  used  for  this  purpose. 

104.  Railway  Motors.  —  Series  motors  operating  on  con- 
stant-potential circuits  of  from  500  to  600  volts  furnish  a 
very  satisfactory  motive  power  for  the  propulsion  of  trolley 
street  cars  and  electric  railway  motor  cars.     This  type  of 


242  DYNAMO   ELECTRIC   MACHINERY. 


2000      100 


1000 


\ 


2  4 

H.  P.  OUTPUT 


Fig.  151. 


40   C 


t 
3 
520 


10 


\ 


400  800 


1200  1600 

REV.  PER  MIN. 


Fig.  152. 


MOTORS.  243 

motor  has  been  developed  to  a  high  degree  of  perfection 
during  recent  years,  and  is  reasonably  well  fitted  to  meet 
the  many  and  severe  requirements  of  railway  service.  Re- 
cent improvements  are  directed  to  reliability  rather  than  to 
increased  efficiency.  It  is  not  unusual  for  modern  railway 
motors  to  be  in  service  for  a  year  or  more  or  to  have  trav- 
eled 60,000  miles  without  overhauling. 

A  railway  motor  must  be  mechanically  strong  to  with- 
stand the  continual  strains  to  which  it  will  be  subjected 
when  in  service.  Poor  roadbed,  defective  switches,  snow- 
covered  tracks,  etc.,  are  conditions  met  with  in  railway  ser- 
vice. Railway  motors  are  also  subject  to  abuse  at  the 
hands  of  the  motormen.  The  series  resistance  is  often  cut 
out  too  rapidly,  before  the  car  has  an  opportunity  to  accel- 
erate. As  a  result  there  is  an  enormous  current  flow  and 
a  large  torque  exerted  with  little  speed.  This  severely 
strains  the  motor  and  is  particularly  liable  to  disturb  the 
armature  windings.  Railway  motors  are  of  weatherproof 
construction,  being  totally  enclosed  to  guard  against  the 
intrusion  of  water,  slush  and  mud. 

Fig.  153  illustrates  the  box-type  frame  of  a  No.  134 
Westinghouse  railway  motor  for  the  heavier  class  of  inter- 
urban  service.  There  are  four  poles  built  up  of  soft  steel 
punchings  assembled  and  riveted  together  between  wrought- 
iron  end  plates  and  bolted  to  the  motor  frame.  The  field 
coils  are  straight  and  are  formed  of  copper  strap  wound  on 
edge,  the  individual  turns  being  insulated  from  each  other 
by  asbestos.  The  coils  are  insulated  by  several  tapings 
and  impregnated  with  an  insulating  compound  to  render 
them  impervious  to  moisture.  They  are  held  in  place 
independently  of  the  poles  by  brass  hangers  which  are 
bolted  to  the  motor  frame. 


244 


DYNAMO   ELECTRIC   MACHINERY. 


The  armature  bearings  of  this  motor  are  carried  in  hous- 
ings which  are  firmly  clamped  into  bored  seats  in  the  frame. 
The  bearing  at  the  commutator  end  is  3|  inches  in  diame- 
ter and  10  inches  long.  One  end  of  the  motor  frame  con- 
tains bearings  which  run  on  the  wheel  axle  and  keep  the 


Fig-  153- 

pitch  circle  of  the  armature  shaft  pinion  always  tangent  to 
the  pitch  circle  of  the  gear  which  is  mounted  on  the  axle. 
These  bearings  are  1 1 J  inches  long  and  are  furnished  for  a 
maximum  axle  diameter  of  6  inches.  The  bearings,  both 
for  armature  shaft  and  for  axle,  consist  of  solid  bronze  shells 
lined  with  babbitt  metal  soldered  to  the  bronze,  and  are 


MOTORS.  245 

arranged  for  oil-saturated  waste  as  lubricant.  Large  pock- 
ets are  provided  for  the  waste  which  is  in  contact  with  the 
shaft,  and  the  oil  is  led  up  to  the  waste  from  below.  The 
openings  in  the  bearing  shells  are  usually  60  %  of  the  total 
length  of  the  shell  and  80  degrees  wide. 

The  armature  is  built  up  of  thin  soft-steel  laminations 
mounted  on  a  spider  together  with  the  commutator.  Open- 
ings in  the  laminations  and  spaces  between  groups  of  them 
provide  for  thorough  ventilation  by  means  of  the  air  drawn 
in  at  the  ends  and  passing  out  against  the  field  windings. 
The  winding  consists  of  formed  single-turn  coils  made  in 
two  parts,  the  lower  and  upper  halves  being  connected  at 
the  rear  of  the  armature  by  soldered  copper  clips.  The 
winding  is  insulated  with  mica  and  sealed  by  linen  tape 
followed  by  dipping  in  varnish  to  render  it  oil-proof.  The 
winding  is  firmly  secured  in  place  by  steel  wire  wound  around 


Fig.  154- 
X 

the  core  and  over  the  end  connections.  Fig.  154  shows  the 
armature  and  commutator  mounted  on  the  shaft.  The 
diameter  of  the  armature  is  17^  inches  and  that  of  the 
commutator  is  14}  inches.  Brush  holders,  Fig.  155,  for 
this  motor  are  supported  by  two  steel  studs  which  are 


246 


DYNAMO   ELECTRIC   MACHINERY. 


secured    to    the    motor    frame    by    means    of    clamps,    as 
shown. 

The  railway  motor  described  in  the  foregoing  has  a 
nominal  rating  of  160  H.P.  based  on  a  one-hour  run  with 
a  temperature  rise  not  exceeding  75°  C.,  as  thermometri- 
cally  measured,  in  any  part  of  the  winding  above  the  sur- 
rounding air  taken  at  25°  C.  An  equipment  comprising 


Fig.  155. 

• 

two  such  motors  would  propel  a  car  weighing,  without  pas- 
sengers and  electrical  equipment,  25  tons,  over  a  level 
track,  and  maintain  a  schedule  speed  of  30  miles  per  hour 
with  stops  two  miles  apart.  These  figures  are  based  upon 
a  gear  ratio  of  24-53  and  33-inch  car  wheels.- 

The  performance  curves  of  this  railway  motor  at  500 
volts  are  given  in  Fig.  156.  The  usual  torque  and  rev.- 
per-min.  curves  of  motors  are  replaced  in  railway  work  by 
curves  of  tractive  effort  (i.e.,  force  exerted  at  the  base  of 
the  car  wheels)  and  speed  in  miles  per  hour.  Knowing 
the  gear  ratio,  gear  efficiency,  /?,  and  car-wheel  diameter, 


MOTORS. 


247 


D  inches,  this  conversion  can  be  effected  by  means  of  the 
following  expressions  : 

Lbs.  Tractive  Effort  = 

no.  gear  teeth        24  S      ~ 

— r    •   "-L^-  X  Torque  in  Ibs.-ft. 
no.  pinion  teeth        D 

, ...,  2  TT  60  Rev.  per  min.  x  Torque  in  Ibs.ft. 

Miles  per  Hour  =  -  — 

5280  X  Lbs.  Tractive  Effort 

The  continuous  capacity  of  this  motpr  is  given    as    120 
amperes  at  300  volts  and  no  amperes  at  490  volts. 


6000 


5000      250 


A   motor  for  railway  service,  very  similar  in  design  to 
the  one  just  described,  is  the  GE-2i6  made  by  the  Gen- 


248  DYNAMO   ELECTRIC   MACHINERY. 


MOTORS. 


249 


era!  Electric  Company,  and  shown  in  Fig.  157.  It  is  a 
4-pole  motor  provided  with  an  equal  number  of  commutat- 
ing  poles,  the  latter  being  conducive  to  better  commutation 
during  the  acceleration  period.  The  one-hour  rating  of 
this  motor  is  50  H.P.  at  600  volts. 

The  gear  case  rides  with  the  motor  and  is  fastened  to  the 
magnet  frame  at  three  points  in  order  to  eliminate  vibra- 
tion. The  case  is  made  of  malleable  iron  and  constructed 
with  strengthening  ribs  to  prevent  cracking. 

Some  operating  characteristics  and  constructive  data  of 
55O-volt  railway  motors  are  embodied  in  the  curves  of  Fig. 
158.  Curve  A  represents  the  efficiency  of  the  various  sizes 


.80     100 


3000     80      10000 


1500      20       7000 


H.  P.  OUTPUT 

Fig.  158. 

of  motors  at  normal  load,  curve  B  shows  the  radial  length 
of  the  air-gap  between  armature  and  field  poles,  curve  C 
gives  the  peripheral  speed  of  the  armature  in  feet  per 
minute,  curve  D  indicates  the  weight  of  the  motor  per 
horse-power,  and  curve  E  shows  the  number  of  ampere- 
turns  per  field  coil  at  normal  load  current. 


250  DYNAMO  ELECTRIC   MACHINERY. 


MOTORS.  251 

The  manner  of  suspending  the  motors  from  the  trucks 
is  a  matter  of  considerable  importance.  One  end  of  the 
motor  frame  contains  bearings  which  run  on  the  wheel 
axle,  and  the  other  end  or  the  sides  are  provided  with  lugs 
for  attachment  to  a  heavy  bar  which  is  supported  by  springs 
on  the  truck  frame.  Figs.  159  and  160  show  two  methods 
of  motor  suspension. 

At  present  a  few  interurban  railways  are  in  operation 
in  this  country  upon  which  1200-  and  i4OO-volt  series 
motors  are  used.  The  design  of  these  motors  is  not  mate- 
rially different  from  that  of  the  6oo-volt  type,  but  particu- 
lar attention  is  directed  to  the  avoidance  of  commutation 
difficulties. 

105.  Railway  Motor  Control.  —  The  function  of  a  rail- 
way controller  is  to  allow  the  motors  to  start  from  rest  and 
to  accelerate  to  full  speed,  this  operation  being  performed 
with  moderate  uniformity,  due  consideration  being  given  to 
the  durability  of  the  apparatus  a,nd  to  the  comfort  of  pas- 
sengers. Two  general  methods  for  accomplishing  this 
result  are  in  use  :  the  rheostatic,  and  the  series-parallel 
method. 


Fig.  161. 

In  the  rheostatic  method  for  use  with  one  or  more  motors, 
resistance  is  placed  in  the  motor  circuits,  which  can  be  ad- 
justed to  regulate  the  impressed  electromotive  force.  A 
scheme  of  connection  for  a  rheostatic  railway  controller  is 
given  in  Fig.  161.  The  change  of  the  resistance  in  the 
motor  circuit  is  accomplished  by  short-circuiting  successive 


252  DYNAMO   ELECTRIC   MACHINERY. 

portions  of  it  by  closing  switches  i,  2,  3  and  4  in  the  order 
named.  This  method  is  infrequently  employed  although 
simple,  because  the  loss  in  the  regulating  resistance  does 
not  permit  of  economical  operation. 

The  series-parallel  method  of  motor  control  is  exten- 
sively used  for  equipments  with  two  (or  any  multiple  of 
two)  motors.  The  speed  of  the  car  is  regulated  by  first 
placing  the  two  motors  and  a  resistance  in  series,  and  then 
cutting  out  the  resistance  step  by  step  until  the  motors 
are  operating  in  series  on  full  voltage.  Since,  with  all  the 
resistance  cut  out,  there  is  no  unnecessary  I2R  loss,  this  is 
called  a  running  connection,  and  the  controlling  mechanism 
is  said  to  be  upon  a  running  point.  To  further  increase 
the  speed,  the  motors  are  placed  in  parallel  with  a  resist- 
ance in  series  with  both.  This  resistance  is  then  cut  out 
step  by  step  until  the  motors  are  each  operating  on  full 
voltage.  This,  again,  constitutes  a  running  connection. 

The  connections  of  a  series-parallel  controller  are  more 
complex  than  those  of  the  rheostatic  type,  since  the  mat- 
ter of  transition  from  the  series  to  the  parallel  positions 
demands  attention.  During  this  period  one  motor  may 
be  shunted  or  short-circuited,  the  motor  circuit  may  be 
opened,  or  the  full  current  may  be  maintained  through  all 
motors.  A  scheme, of  connections  illustrating  the  latter 
type  of  series-parallel  control  is  shown  in  Fig.  162.  The 
controller  performs  the  following  operations :  switches 
A  and  B  are  closed,  thus  placing  both  motors  and  all  the 
resistance  in  series ;  switches  i  to  7  are  closed  consecu- 
tively and  then  switch  C  is  closed ;  followed  by  the  opening 
of  switches  2  to  7  and  B ;  switches  a  and  b  are  closed  ;  thus 
two  currents  will  flow  through  switch  C  in  opposite  direc- 
tions, one  from  the  trolley  through  the  motors  to  ground 


MOTORS.  253 

and  the  other  through  the  resistance  to  ground.  If  the 
resistance  be  properly  proportioned  no  current  will  pass 
through  switch  C,  and  this  may  be  opened,  thus  placing 
both  motors  with  resistance  directly  across  the  line. 
Switches  2  to  7  are  then  closed  progressively  as  before, 
after  which  the  motors  are  operating  in  parallel  without 
resistance.  This  scheme  is  therefore  desirable  in  that 
no  motor  is  subjected  to  sudden  increased  voltage  nor  is 
the  circuit  opened  at  any  time. 


Fig.  162. 

When  four  motors  are  installed  on  a  car,  they  may 
first  be  connected  in  series,  then  each  pair -in  parallel 
with  the  two  groups  in  series,  and  finally  all  connected  in 
parallel ;  this  is  known  as  the  series,  series-parallel,  parallel 
method. 

The  manipulation  of  the  various  switches  may  be  accom- 
plished directly  by  hand  or  through  the  intervention  of  an 
auxiliary  control.  In  the  former  system  the  connections 
are  made  by  a  motorman  who  moves  a  handle  at  the  top  of 
the  controller  on  the  car  platform.  This  causes  the  rota- 
tion of  a  vertical  cylinder  and  permits  of  the  successive 
connection  of  various  contact  studs  thereon  with  stationary 
fingers.  Such  a  controller,  made  by  the  Westinghouse 
Electric  and  Manufacturing  Company,  is  shown,  with  the 


254 


DYNAMO   ELECTRIC    MACHINERY. 


cover  removed,  in  Fig.  163.  In  this  controller  for  series- 
parallel  operation  there  are  seven  controlling  points  in  the 
series  position  and  six  in  the  parallel  position ;  during  the 
transition  from  one  position  to  the  other  one  of  the  motors  is 
short-circuited.  The  wires  frorr  the  trolley,  from  the 


Fig.  163. 

motors,  and  from  the  different  terminals  of  the  resistance 
grids  are  brought  up  under  the  car  to  the  proper  ringers. 
A  smaller  cylinder,  moved  by  a  reversing  lever,  is  situated 
to  the  right  of  the  main  cylinder.  This  has  contact 
pieces  which  are  arranged  so  as  to  enable  the  motorman 
to  reverse  the  direction  of  rotation  of  both  motors  or 
to  cut  them  out  entirely.  Interlocking  devices  are  sup- 


MOTORS.  255 

plied  so  that  the  reversing  handle  cannot  be  moved  unless 
the  controlling  handle  is  in  such  a  position  that  connec- 
tion with  the  trolley  is  broken.  The  controlling  handle  also 
cannot  be  moved  if  the  reversing  handle  is  not  properly 
set  either  to  go  forward  or  to  go  backward.  The  reversing 
handle  cannot  be  removed  from  the  controller,  save  when 
the  smaller  cylinder  is  in  the  position  that  cuts  out  both 
motors.  As  serious  arcs  are  liable  to  develop  upon  break- 
ing a  circuit  of  500  volts,  the  contact  pieces  and  fingers  are 
separated  from  adjacent  ones  by  strips  of  insulating  mate- 
rial which  are  fastened  to  the  inside  of  a  separate  cover. 
Such  arcs  are  effectively  disrupted  by  the  field  of  an  electro- 
magnet, which  is  an  essential  part  of  controllers  for  motors 
of  large  size. 

In  operating  an  electric  car,  the  power  should  never  be 
turned  off  by  a  slow  reverse  movement  of  the  controller 
handle,  as  destructive  arcs  are  liable  to  occur  upon  a  slow 
break.  To  lower  the  speed  of  a  car,  the  power  should  be 
completely  and  suddenly  shut  off.  Before  the  car  has 
slackened  its  speed  too  much  the  controller  handle  can  be 
brought  up  to  the  proper  point. 

The  system  of  motor  car  control  in  which  the  various 
switches  are  operated  by  an  auxiliary  circuit  is  called  the 
multiple-unit  control,  since  it  is  designed  primarily  for 
the  operation  of  several  cars  coupled  together  in  a  train 
from  any  controller  on  it.  The  control  apparatus  for  each 
motor  car  consists  essentially  of  a  series-parallel  motor  con- 
troller and  two  master  controllers.  The  motor  controller 
is  composed  of  a  number  of  electrically  operated  switches 
or  contactors  which  close  and  open  the  various  motor  and 
resistance  circuits,  and  a  separate  electrically  operated  re- 
versing switch  which  governs  the  direction  of  movement 


256  DYNAMO   ELECTRIC   MACHINERY. 

of  the  car.  This  apparatus  is  usually  placed  underneath 
the  car.  Both  the  contactors  and  the  reverser  are  operated 
by  solenoids,  the  current  to  which  is  varied  by  the  master 
controller.  The  latter  is  considerably  smaller  than  the 
ordinary  street-car  controller,  but  is  similar  in  appearance 
and  method  of  operation.  A  cable  which  connects  each 
master  controller  with  the  motor  controllers  runs  the  en- 
tire length  of  the  train,  the  connections  between  cars  being 
made  by  suitable  couplers.  Current  for  the  master  con- 
trol is  taken  from  the  line  through  whichever  controller 
the  motorman  operates,  the  amount  being  about  2j  am- 
peres for  each  equipment  of  400  H.P.  As  the  motor 
controller  is  connected  to  the  train  auxiliary  circuit,  any 
master  controller  on  the  train  will  simultaneously  operate 
corresponding  contactors  on  all  the  motor  cars  and  estab- 
lish similar  motor  connections  on  them.  The  connections 
of  the  Sprague-General  Electric  multiple-unit  control  sys- 
tem are  shown  in  Fig.  164. 

If  the  current  supply  be  momentarily  interrupted,  the 
motor  control  switches  automatically  return  to  the  "off" 
position,  and  upon  the  restoration  of  the  power  supply 
the  same  connections  are  again  progressively  made  that 
existed  immediately  preceding  the  interruption.  To 
avoid  accidents  which  may  occur  through  the  physical 
disability  of  a  motorman,  master  controllers  are  sometimes 
arranged  with  a  button  on  the  handle  which  must  be 
kept  down  in  order  to  keep  the  auxiliary  control  circuit 
closed. 

The  multiple-unit  control  system  of  the  Westinghouse 
Electric  Company  differs  from  the  preceding  in  the  method 
of  actuating  the  contactors  and  reversers.  Compressed 
air  is  used  for  this  purpose,  the  necessary  valves  being 


MOTORS. 


257 


258  DYNAMO   ELECTRIC  MACHINERY. 

operated  electrically  by  a  master  controller  from  a  storage 
battery. 

106.  Motors  for  Automobiles.  —  For  electric  automo- 
biles the  series-wound  motor  is  invariably  employed.  A 
storage  battery  of  40  or  44  cells  is  the  customary  source 
of  power  for  these  motors.  The  use  of  these  cells  affords 
a  convenient  and  economical  means  of  speed  control.  In 
the  case  of  a  single  motor,  for  the  first  controller  notch, 
the  cells  may  be  connected  in  four-series  groups  of  10  or 
1 1  each,  giving  about  22  volts,  the  four  groups  being  con- 
nected in  parallel.  Other  notches  would  correspond  to 
other  series-parallel  combinations,  and  finally  the  last  and 
highest  speed  notch  would  correspond  to  a  connection  of 
all  the  cells  in  series.  By  this  arrangement  one  cell  is 
used  just  as  much  as  any  other,  and  they  are  discharged 
at  equal  rates.  As  the  voltage  supplied  to  the  motor  is 
varied  without  recourse  to  a  series  regulating  resistance, 
there  is  no  resistance  loss  in  starting  or  running  at  less 
than  full  speed. 

Frequently  two  motors  are  used,  one  on  each  of  the 
two  driving  wheels ;  this  arrangement  allows  independent 
rotation  of  the  wheels  on  turning  curves,  while  if  only  one 
motor  be  used  some  form  of  differential  gear  must  be  em- 
ployed to  allow  the  vehicle  to  make  sharp  turns.  But  the 
efficiency  of  one  motor  is  in  general  greater  than  the  effi- 
ciency of  two  motors  of  half  the  rating,  and  the  gain  in 
efficiency  by  using  one  motor  more  than  balances  the  cost 
and  complication  of  a  differential  gear  in  the  case  of  light 
vehicles. 

It  is  general  practice  to  rate  automobile  motors  at  75 
volts,  or  at  37 \  volts  if  two  are  used  in  series  and  con- 
trolled as  a  single  motor.  Since  the  voltage  of  40  or  44 


MOTORS.  259 

cells  of  battery  in  series  can  fall  to  75  volts  without  injury, 
this  is  the  lowest  pressure  on  which  the  motors  will  be 
expected  to  run  for  any  length  of  time  at  full  speed. 
Hence  this  voltage  is  used  as  the  basis  for  rating.  For 
the  best  motors  the  rating  is  such  that  a  temperature  rise 
of  50°  C.  will  not  be  exceeded  on  an  indefinite  run.  A 
motor  so  rated  will  carry  100  per  cent  overload  for  a  half 
hour  without  overheating  or  damaging  the  insulation. 

Although  the  voltage  of  these  motors  is  somewhat  low 
for  the  use  of  carbon  brushes,  the  necessity  of  reversal  of 
direction  and  the  liability  of  sparking  on  overload  make 
their  use  desirable.  Soft  carbon  brushes  of  low  resistance 
can,  however,  be  obtained,  and  they  are  to  be  recom- 
mended. 


Fig.  165. 

Fig.  165  illustrates  a  motor  which  is  used  on  automo- 
biles and  manufactured  by  the  Eddy  Electric  Manufactur- 
ing Company.  The  armature  winding  consists  of  formed 
coils  which  are  cross-connected,  and  therefore  only  two 
brushes  are  required.  These  brushes  are  made  accessible 
by  providing  a  window  in  the  end  plate.  A  pinion  which 


260  DYNAMO   ELECTRIC   MACHINERY. 

is  mounted  upon  the  armature  shaft  meshes  with  an  inter- 
nal gear  on  the  wheel  of  the  vehicle. 

107.  Motors  for  Rolling  Mills.  —  For  many  kinds  of 
mill  work  requiring  great  torque,  reversibility,  and  wide 
variation  of  speed,  the  series  motor  is  well  adapted.  The 
shocks  and  jars  which  such  motors  must  withstand  are 
very  severe  because  the  load  conditions  are  heavy  and 
intermittent,  and  therefore  they  must  be  of  particularly 
strong  construction.  Mill  motors  must  be  totally  enclosed 
to  guard  against  dust  and  small  particles  of  metal,  and  con- 
sequently must  be  amply  designed  so  that  their  tempera- 
ture elevation  will  not  be  excessive.  These  motors  usually 
operate  in  both  directions  and  therefore  the  brushes  can 
have  no  lead.  Sparkless  running  is  insured  by  employing 
large  air-gaps. 

The  Crocker- Wheeler  Company  manufacture  form  W 
motors  for  rolling  mills  in  sizes  ranging  from  J\  to  200 
H.P.  for  220  volts,  one  of  which  is  shown  in  Fig.  166. 
They  are  four-pole  machines,  and  the  frames  are  divided 
horizontally,  the  upper  half  being  provided  with  two  hand 
holes  for  access  to  the  commutator  and  brushes. 


w 


TWO  HIGH 
MILL 

THREE  HIGH 
MILL 


Fig.  166.  Fig.  167. 

A  rolling  mill  may  be  two  high  or  three  high,  as  indi- 
cated in  Fig.    167.     In  the  latter  the  center  roll  rotates 


MOTORS  26l 

constantly  in  one  direction,  while  the  other  two  rolls  revolve 
constantly  in  the  opposite  direction.  Thus  a  piece  of  steel 
can  pass  through  the  lower  set  to  the  right,  then  be  raised 
on  a  table  and  pass  through  the  upper  set  of  rolls  to  the 
left,  and  continuing  in  this  way.  In  the  two-high  mill  the 
steel  must  pass  through  the  rolls  in  both  directions  and 
consequently  the  motor  driving  such  a  mill  must  be  capa- 
ble of  reversal.  Difficulty  is  encountered  in  designing 
motors  which  are  to  reverse  their  direction  quickly  because 
of  the  large  moments  of  inertia  of  the  armatures  and 
rolls.  Sometimes  two  or  three  armatures  of  smaller 
diameter  than  an  equivalent  single  armature  are  placed 
upon  one  shaft,  thus  obtaining  a  smaller  radius  of  gyra- 
tion. 

A  mill  motor  has  been  built  by  Messrs.  Siemens  weigh- 
ing 235  tons,  the  armature  weighing  74  tons,  and  it  is 
capable  of  exerting  a  torque  of  650,000  Ibs.-ft.  up  to  a 
speed  of  60  rev.  per  min.,  thus  corresponding  to  over  7000 
H.P.  output.  This  mill  has  been  in  operation  for  some 
time,  and  it  is  found  possible  to  reverse  its  direction  28 
times  per  minute  from  a  speed  of  60  rev.  per  min.  in  one 
direction  to  an  equal  speed  in  the  other  direction. 

The  coupling  between  a  motor  and  a  rolling  mill  should 
be  such  that  if  the  roll  breaks  obliquely  the  resulting  end 
thrust  will  not  damage  the  motor.  Some  form  of  shell 
coupling  should  be  used  between  the  spindle  and  the  motor 
shaft. 

108.  Crane  Motors.  —  Series  motors  for  operating  cranes 
or  hoists  are  generally  equipped  with  a  brake  attachment 
so  that  the  load  may  be  held  after  raising  it.  Brakes  are 
of  two  types,  friction  brakes  and  dynamic  brakes.  Fric- 
tion brakes  are  made  in  a  number  of  ways,  one  type  of 


262  DYNAMO   ELECTRIC   MACHINERY. 

which  is  depicted  in  Fig.  168.  This  shows  a  spring-actu- 
ated shoe  brake  which  is  kept  normally  in  engagement  but 
is  released  when  current  is  supplied  to  the  solenoid.  An- 
other form  is  the  band  brake,  but  this  is  mostly  used  with 
non-reversing  motors,  although  some  forms  are  applicable 
to  reversing  motors. 


Fig.  168. 


Dynamic  braking  is  accomplished  by  connecting  the 
motor  to  operate  as  a  generator  which  will  deliver  energy 
to  some  local  circuit  or  return  it  to  the  supply  circuit. 
Such  brakes  are  generally  supplemented  by  friction  brakes 
which  become  operative  when  the  motor  comes  to  rest  and 
the  dynamic  braking  ceases.  The  controller  for  dynamic 
braking  is  arranged  to  connect  the  armature  in  a  closed 
circuit  containing  an  adjustable  resistance,  or  to  the  line. 
It  is  desirable  first  to  connect  the  series  field  with  resist- 
ance across  the  line  wires  to  insure  the  motor  building  up 
as  a  series  generator.  Then  the  motor  is  disconnected 


MOTORS. 


263 


from  the  line,  leaving  the  motor  armature  and  field  in  cir- 
cuit with  the  resistance.  In  Fig.  169  these  connections 
are  shown  respectively  at  a  and  b. 


RESISTANCE 


Fig.  169. 

A  controller  for  crane  motors  made  by  the  Cutler- 
Hammer  Manufacturing  Com- 
pany, intended  for  either  hoist 
or  travel  duty,  is  shown  in  Fig. 
170.  The  resistances  are  of 
the  cast-iron  grid  type  and  are 
supported  by  iron  rods  over 
which  mica  tubes  are  previously 
placed.  Controllers  for  hoist 
duty  are  provided  with  higher 
resistance  than  those  for  travel 
duty  so  as  to  insure  good 
speed  control  under  light 
loads. 

109.  Compound -wound  Mo- 
tors. —  In  a  compound-wound 
motor  the  magnetomotive  force 
of  the  shunt  winding  may  as- 
sist or  oppose  the  magneto- 
motive force  of  the  series 
winding,  depending  upon  the 
manner  of  connection.  In  the 
first  case  the  machine  is  called  Fie-  IT°- 

a  compound  motor  and  in  the  latter  a  differential  motor. 


264  DYNAMO   ELECTRIC   MACHINERY. 

The  magnetic  field  of  a  compound  motor  becomes  more 
intense  with  increasing  load,  and  consequently  the  speed 
decreases  ;  the  amount  of  speed  decrease  will  depend  upon 
the  relative  magnetomotive  forces  of  the  two  windings. 
At  light  load  there  is  always  a  definite  field  strength,  due 
to  the  shunt  winding,  and  therefore  the  speed  of  the  motor 
cannot  exceed  a  predetermined  value.  For  heavy  inter- 
mittent loads,  such  as  in  operating  rolling  mills,  hoists,  ele- 
vators, etc.,  compound  motors  are  much  used,  because  they 
can  exert  a  powerful  starting  torque  and  yet  the  speed  will 
not  be  excessively  variable  under  changes  of  load.  Such 
motors  may  be  safely  belted  to  machine  tools  in  operating 
them. 

Differential  motors  may  be  designed  to  run  at  almost  con- 
stant speed  by  properly  proportioning  the  series  and  shunt 
windings  so  that  the  magnetic  field  becomes  weaker  as 
the  load  increases.  A  powerful  starting  torque  cannot  be 
obtained  from  this  motor  inasmuch  as  the  large  starting 
current  in  the  series  winding  greatly  decreases  the  field 
strength.  If  such  motors  be  suddenly  started  under  load 
their  direction  of  rotation  may  reverse  because  the  series 
winding  has  a  lower  inductance  than  the  shunt  winding ; 
hence  in  starting  differential  motors  the  series  winding  at 
first  should  be  automatically  short-circuited.  These  motors 
are  rarely  used  in  practice,  as  improvements  in  the  design 
of  shunt  motors  have  given  the  latter  good  speed  regula- 
tion, so  there  is  no  need  of  resorting  to  differential  motors 
with  their  objectionable  features. 


PROBLEMS.  265 

PROBLEMS. 

1.  The  armature  core  of  a  4-pole  motor  has  41  slots,  each 
containing  24  conductors.     At  full  load  the  wave-wound  arma- 
ture takes  20  amperes  at  500  volts,  and  a  flux  of  2,100,000 
maxwells  enters  the  armature  core  per  pole.     What  torque  is 
developed  at  full  load  ? 

2.  If   the    armature    mentioned    in   the   preceding   problem 
revolve  at  640  rev.  per  min  ,  determine  the  horse-power  devel- 
oped at  full  load. 

3.  The  armature  resistance,  including  that  of  brushes  and 
brush  contacts,  of  a  25o-volt,  6-pole  motor  is  .0079  °nm«     The 
armature  is  lap-wound  and  has  570  conductors.     What  is  the 
flux  per  pole  entering  the  armature  when  the  latter  runs  at  400 
rev.  per  min.  and  takes  660  amperes  at  full  load  ? 

4.  A  25-H.P.,  22o-volt  shunt-wound  motor  takes  a  current 
of  98  amperes  at  full  load.     The  armature  resistance  is  0.090 
ohm.     With  a  maximum  allowable  current  intake  of  1 40  amperes, 
calculate  the  number  of  steps  required  in  a  starting  rheostat  for 
this  motor. 

5.  What  are  the  resistance  values  of  the  various  steps  of  the 
starting  box  mentioned  in  the  previous  problem  ? 

6.  From   the  following  name-plate  data  of    a   shunt  motor 
determine  its  regulation  and  full-load  efficiency : 

H.P.  =  50,         Volts  =  250,          Amperes  =170, 
R.P.M.  =  420  at  no  load,      R.P.M.  =  400  at  full  load. 

7.  What  is  the  average  pull  in  pounds  on  each  of  the  372 
armature  conductors  of  the  motor  cited  in  the  foregoing  problem 
at  full  load  if  the  conductors  are  situated  9  inches  from  the  axis  ? 

8.  The  motor  of  an  electric  car  having  33-inch  wheels,  when 
traveling  at  25  miles  per  hour,  exerts  a  torque  of  550  pounds  (at 
one  foot  radius  from  the  center  of  the  armature  shaft).    If  the  gear 
ratio  is  26  to  60,  and  the  efficiency  of  the  gears  is  97%,  determine 
the  tractive  effort  at  the  base  of  the  car  wheels,  the  horse-power, 
and  the  number  of  revolutions  of  the  motor  per  minute. 


266  DYNAMO    ELECTRIC   MACHINERY. 


CHAPTER    VIII. 

DYNAMOTORS,    MOTOR-GENERATORS,     BOOSTERS,     AND 
STORAGE    BATTERIES. 

no.  Dynamotors.  —  Dynamotors  are  transforming  de- 
vices combining  both  motor  and  generator  action  in  one 
magnetic  field,  with  two  armatures  or  with  an  armature 
having  two  separate  windings.  They  are  generally  supplied 
with  commutators,  one  at  each  end,  which  are  connected  to 
the  two  windings  respectively.  Either  winding  of  the  ar- 
mature may  be  used  as  a  motor  winding,  and  the  other  as 
the  generator  winding.  These  machines  occupy  the  same 
position  as  regards  direct-current  practice  as  is  occupied 
by  transformers  in  alternating-current  practice.  That  is, 
they  enable  one  to  take  electrical  energy  from  a  system  of 
supply  at  one  voltage,  and  deliver  it  at  another  voltage  to 
a  circuit  where  it  is  to  be  utilized.  They  cannot,  however, 
be  constructed  so  as  to  operate  with  the  same  high 
efficiency  as  a  transformer  does.  As  the  currents  in  the 
two  armatures  flow  in  opposite  directions,  and  the  machines 
are  so  designed  as  to  have  practically  the  same  number  of 
armature  ampere-turns  when  in  operation,  there  is  practi- 
cally no  armature  reaction.  The  field,  therefore,  is  not 
distorted  so  as  to  require  a  shifting  of  the  brushes  upon  a 
change  of  load.  These  machines  are  more  efficient  than 
motor-generators,  which  will  be  described  later,  as  they 
have  but  a  single  field  magnet.  They  cannot  be  com- 
pounded so  as  to  yield  a  constant  E.M.F.  at  the  dynamo 
end,  for,  while  a  cumulative  series  coil  would  tend  to  raise 


DYNAMOTORS.  267 

the  E.M.F.  at  the  generator  end,  there  would  be  a 
corresponding  decrease  of  the  speed  of  the  armature, 
due  to  the  increase  of  magnetic  flux.  To  generate  the 
same  counter  E.M.F.  in  the  motor  winding  as  previously 
existed  without  the  series  coil,  requires  a  lower  armature 
speed. 

Dynamotors  are  used  in  the  so-called  Teaser  system  of 
variable  speed  control  of  motors  which  are  intended  to  re- 
ceive energy  from  supply  circuits,  that  are  at  the  same  time 
giving  energy  to  lighting  and  similar  load  circuits.  For 
instance,  they  are  used  in  connection  with  large  printing 
presses  whose  parts  have  large  moments  of  inertia,  and  which 
demand  an  unusually  large  starting  torque  to  be  exerted  by 
the  driving  motor.  Sometimes  this  is  as  much  as  six  times 
the  torque  which  the  motor  is  called  upon  to  exert  at  nor- 
mal speed  and  full  load.  Since  the  torque  which  is  exerted 
by  a  motor  is  dependent  upon  the  current  which  flows 
through  its  armature,  while  the  speed  at  which  this  torque 
is  applied  is  dependent  upon  the  pressure  impressed  upon 
it,  it  is  desirable  that  the  large  starting  current  should  be 
taken  at  a  low  voltage  from  some  transforming  device,  such 
as  a  dynamotor,  rather  than  at  the  higher  and  constant  volt- 
age of  the  supply  mains.  The  dynamotor  for  instance 
may  be  so  designed  that  the  counter  E.M.F.  of  its  motor 
winding  is  five  times  the  E.M.F.  induced  in  its  generator 
winding.  Consider  the  dynamotor  and  main  motor  to  be 
connected  to  the  supply  mains  as  indicated  in  Fig.  171,  the 
field  winding  of  the  former  being  excited  by  current  taken 
directly  from  the  mains,  and  the  negative  brush  of  the 
motor  end  being  connected  with  the  positive  brush  of  the 
generator  end.  The  two  armature  windings  are  connected 
in  series  with  a  regulating  resistance  to  the  supply  mains. 


268 


DYNAMO   ELECTRIC   MACHINERY. 


WWWVWAA 


TEAZER 
ARMATURE 


'WWWA/VXJ 


At  starting,  the  main  motor,  which  drives  the  press  and 
which  is  generally  a  cumulatively  compound -wound  motor, 
is  supplied  with  current  from  the  generator  end  of  the  dyna- 

_  motor.  The  voltage  with 
which  it  is  supplied  is  some- 
what less  than  one-fifth  that 
of  the  main  supply,  depending 
upon  the  magnitude  of  the 
resistance  in  series  with  the 
dynamotor.  This  low  voltage 
permits  of  the  application  of  a 
proper  amount  of  torque  at  a 
low  speed.  Furthermore,  the 
drain  of  current  from  the  sup- 
ply mains  is  about  one-fifth 
Flg'  I?I'  that  which  passes  through 

the  main  motor.  By  manipulating  the  dynamo  regulating 
resistance,  the  electromotive  force  supplied  to  the  main 
motor  is  raised,  and  with  it  the  speed.  The  highest 
speed  of  the  main  motor  which  can  be  attained  by  this 
arrangement  is  such  that,  when  attained,  the  motor's 
connections  may  be  transferred  to  the  supply  mains 
through  another  series  regulating  resistance  without  any 
excessive  drain  of  current  from  those  mains.  The  amount 
of  current  which  is  taken  by  the  main  motor  as  compared 
with  the  amount  of  current  which  is  drawn  from  the  supply 
mains  may  be  represented  as  in  Fig.  172.  Regulation  of 
the  resistances  and  changes  of  connection  are  accomplished 
through  the  aid  of  a  controller.  The  different  speeds  are 
secured  by  the  manipulation  of  a  single  hand-wheel  on  the 
controller,  and  thus  the  pressman  has  at  his  command  a 
simple  means  of  manipulating  the  printing  press. 


DYNAMOTORS. 


269 


Dynamotors  are  also  used  as  equalizers,  in  connection 
with    3-wire    constant-potential    distribution    circuits,     to 


J          AM  PERES  .TAKE  N-FROM^LINE  < 


ASER  CONTROL 


CO  RESI  TANCE 


TIME 

Fig.  172. 


equalize  the  potential  differences  between  the  two  outer 
and  the  neutral  wire  when  one  side  of  the  system  is  carry- 
ing a  larger  load  than  the  other.  For  instance,  with  simi- 


Fig.  173. 


larly  wound  windings  upon  its  armature,  the  dynamotor 
would  be  connected  as  indicated  in  Fig.  173  to  a  system 
supplied  by  a  2 20- volt  generator.  When  the  system  was 


270 


DYNAMO   ELECTRIC    MACHINERY. 


unbalanced  the  side  having  the  smaller  load  would  have 
the  lesser  drop  and  therefore  the  higher  pressure.  The 
winding  of  the  dynamotor  which  is  connected  with  this 
side  would  act  as  a  motor  winding  and  drive  the  armature, 
while  the  other  winding  would  be  the  seat  of  generated 
E.M.F.'s  which  would  tend  to  raise  the  pressure  of  the 
more  heavily  loaded  side. 

Consider  that  each  of  the  equalizer  armature  windings 
has  a  resistance  of  R  ohms  and  generates  Ee  volts,  and 
that  the  power  losses  due  to  hysteresis,  eddy  currents,  and 
friction  are  PI  watts.  Then,  with  E  volts  between  the 
outer  wires,  and  the  load  on  the  two  sides  balanced,  that 
is,  no  current  flowing  through  the  neutral  wire, 

E  =  2Ee+2lQR,  (i) 

and 

Pi  =  2  £/o>  (2) 

where  70  represents  the  current  in  each  armature  winding. 
If,  however,  the  load  be  not  balanced,  and  the  neutral  wire 
carry  a  current  7W,  and  if  the  armature  windings  carry 


fe 


5! 

B                             J| 

c 

•      ) 

X=f— 

Fig.  174. 

currents  II  and  72  respectively,  their  directions  being  as 
indicated  in  Fig.  174,  then 

E  =  2Ee+IiR~  I2R,  (3) 

(§  5)  In   =  A  +  If  (4) 


DYNAMOTORS.  271 

and,  as  the  iron  and  friction  losses  remain  unaltered  upon 
the  changed  conditions  of  load,  and  since  the  power  ex- 
erted by  the  motor  element  equals  the  power  output  of 
the  generator  element  plus  the  frictional  and  iron  losses, 

Ell  =  EJt  +  Pi.  (5) 

Dividing  by  Ee  and  using  (2), 


(6) 


Substituting  (6)  in  (4)  and  solving 


*  (7) 

and  7i  =  LL+  /Q.  (8) 

From  (3),  (7),  and  (8) 

E  =  2  Ee  +  2  IQR,  (9) 

which  is  the  same  as  equation  (i).  Therefore  the  E.M.F. 
generated  by  each  armature  winding  is  not  altered  by  the 
application  of  the  load,  under  the  conditions  assumed. 
Half  of  the  unbalanced  portion  of  the  load  In  is  supplied 
by  one  equalizer  armature  winding,  while  the  other  half 
comes  from  the  main  generator  after  passing  through  the 
other  equalizer  armature  winding,  which  at  the  time  is 
operating  as  a  motor  winding.  The  E.M.F.  's  generated 
in  the  equalizer  windings  are  not  altered  by  the  applica- 
tion of  the  load,  nor  is  the  speed  of  the  armature.  The 
capacity  of  the  equalizer  armature,  if  the  losses  be  disre- 
garded, should  be  equal  to  the  power  represented  by  the 
maximum  unbalanced  load,  for  each  armature  supplies  a 
voltage  approximately  equal  to  that  of  either  side  of  the 


2/2  DYNAMO   ELECTRIC    MACHINERY. 

distribution  circuit,  and  carries  a  maximum  current  of 
approximately  half  the  maximum  carried  by  the  neutral 
wire.  The  employment  of  an  equalizer  enables  one  to 
dispense  with  the  neutral  wire  between  the  point  where 
it  is  located  and  the  main  generator,  and  makes  it  possible 
to  use  a  3-wire  distributing  system  in  connection  with  a 
single  generator  instead  of  a  2-wire  system. 

Assuming  the  normal  pressure  per  side  to  be  half  that 
between  outer  wires,  and  since  with  the  maximum  unbal- 
anced load  the  side  pressures  are  respectively 


=  Ee  +  Ifi, 
arid  EEC  =  Ee  -  I2R, 

then  the  pressure  regulations,  under  this  condition,  are 
+  IJt/Ee  and  —J^R/Ee.  The  pressure  on  the  loaded  side 
of  the  system  therefore  drops  and  that  on  the  unloaded  side 
rises,  as  is  the  case  with  two  main  generators. 

Dynamotors  are  used  in  telegraph  stations,  the  motor 
windings  being  designed  for  connections  to  lighting  cir- 
cuits, while  the  generator  windings  yield  pressures  suitable 
for  telegraphic  service.  This  is  often  different  in  the  case 
of  different  machines.  These  machines  are  designed  to 
take  the  place  of  batteries  of  a  large  number  of  gravity  cells 
such  as  were  used,  in  large  quantities,  a  few  years  ago.  The 
cost  of  operation  of  a  dynamotor  for  this  service  is  about 
one-fifth  of  what  it  is  in  the  case  of  the  gravity  cells.  The 
space  which  the  machine  occupies  is  much  less  than  that  by 
the  cells.  Dynamotors  are  to  be  preferred  to  batteries  also 
on  the  ground  of  cleanliness.  Their  reliability,  when  supplied 
with  electric  energy  from  large  city  service  mains,  is  equal  to 
that  of  the  cells,  but  this  cannot  be  said  in  the  case  of 
small  towns.  The  telephone  companies  sometimes  employ 


MOTOR-GENERATORS. 


273 


dynamotors  for  the  purpose  of  charging  storage  cells. 
Such  a  machine  is  shown  in  Fig.  175.  With  some  forms, 
the  charging  of  the  cells  can  go  on  continuously,  they 
being  at  the  same  time  used  for  telephonic  communication. 
Dynamotors  also  furnish  a  convenient  and  satisfactory 


Fig.  175- 


means  of  heating  surgeons'  electro-cauteries.  Cautery 
knives  take  from  3  to  8  amperes  at  5  volts,  while  dome 
cauteries  take  from  15  to  20  amperes  at  the  same 
voltage. 

in.  Motor-Generators.  —  A  motor-generator  is  a  trans- 
forming device  consisting  of  a  motor  mechanically  connected 
to  one  or  more  generators. 


274 


DYNAMO   ELECTRIC   MACHINERY. 


A  form  in  which  both  the  motor  and  the  generator  are 
direct-current  machines  is  extensively  used  in  connection 
with  3-wire  distributing  systems,  under  the  name  of  bal- 
ancer. If  the  shunt  field  coils  of  the  two  halves  of  the 
balancer  be  connected  in  series  with  each  other  and  to  the 
outer  conductors  of  the  system,  and  if  the  armature  wind- 
ings be  alike,  the  balancer  will  operate  exactly  as  would  a 
dynamotor  when  used  for  this  purpose  and  in  the  manner 
described  in  the  preceding  section.  The  balancer,  however, 
as  well  as  the  motor-generator  in  general,  is  to  be  preferred 
to  the  dynamotor  from  an  operating  viewpoint,  because  the 
absolute  independence  of  the  two  field  magnets  and  of  their 
exciting  coils  makes  possible  a  considerable  variation  in  the 
speed  of  the  motor  element  as  well  as  in  the  voltage  of  the 
generator  element.  If  the  field  windings  of  the  two  ele- 
ments of  the  balancer  and  their  armatures  be  reciprocally 
connected  with  opposite  sides  of  the  system,  as  indicated 
in  Fig.  176,  the  regulation  of  the  voltage  on  the  two  sides 

—  of  the  system  will  be 
improved.  The  E.M.F.'s 
produced  in  the  two  ar- 
matures when  one  side 
of  the  system  is  carry- 
_  ing  a  greater  load  than 
the  other  will  be  unlike, 
the  motor  flux  will  be  reduced  and  the  generator  flux  will  be 
increased.  The  motor  will  therefore  increase  its  speed 
and  the  generator  will  produce  a  greater  E.M.F.  not  only 
because  of  the  increased  speed  of  the  armature  but  also 
because  of  the  increased  flux  from  its  field  magnets. 
With  such  reciprocal  field  excitation  perfect  voltage  regu- 
lation cannot  be  obtained,  for  the  method  postulates  a 


J76. 


MOTOR-GENERATORS. 


275 


voltage  difference  as  a  basis  for  its  operativeness.  Perfect 
regulation  may,  however,  be  obtained  by  the  use  of  com- 
pound-wound elements  in 
the  balancer.  The  direc- 
tions of  the  currents  in  the 
system  and  in  the  exciting 
coils  are  indicated  in  Fig. 
177,  where  the  motor  and 
generator  armatures  of  the 
balancer  are  marked  re- 
spectively with  M  and  G.  The  iron  of  the  magnetic  cir- 
cuits of  the  two  elements  of  the  compound  balancer,  under 
normal  operation,  should  not  be  fluxed  to  near  the  point  of 
saturation. 


Fig.  177. 


Fig.  178. 


A  balancer,  for  use  on  3-wire  circuits  having  a  pressure 
of  125  volts  per  side  is  shown  in  Fig.  178. 


276 


DYNAMO   ELECTRIC   MACHINERY. 


Motor-generators  having  the  armatures  of  their  elements 
wound  for  unlike  voltages  are  used  for  maintaining  con- 
stancy between  the  various  conductors  of  multivoltage 
circuits  as  used  in  connection  with  the  operation  of  variable- 
speed  shunt  motors.  Consider  such  a  balancer  to  be  con- 
nected to  a  circuit  and  the  currents  to  be  as  indicated  in  Fig. 
1 79  and  the  armature  of  the  lower  element  normally  to  gen- 
erate an  E.M.F.  of  E'  volts,  to  have  a  resistance  of  R  ohms, 


Fig.  179. 

and  the  armature  of  the  upper  element  to  generate  (n  —  i)E 
volts  and  to  have  a  resistance  of  (n  —  i)  R  ohms  resistance. 
Then  when  the  load  is  so  balanced  that  the  middle  wire 
carries  no  current,  both  elements  will  act  as  motors  and 
will  carry  a  current  70,  and  the  following  relations  will  hold  : 

E  -  nE'  -  I0nR  =  o.  (i) 

F>       E       TK 
••     E  -'n-1^'  ^ 

When,  again,  the  middle  wire  carries  a  current  /„,  as  indi- 
cated in  the  figure, 

E  -  nE'  -  I^n  -  i)R  +  I'R  =  o,  (3) 

and  (§5)  A  +  /'  =  In,  (4) 

and,  since  motor-element  output  equals  the  sum  of  power 
generated  and  of  the  frictional  losses, 


MOTOR-GENERATORS. 


277 


but,  since  nE'  is  practically  equal  to   E,  dividing  by  E', 
solving  for  /',  and  substituting  in  (4),  there  results 

In 

n 

Substituting  (6)  in  (4), 


(6) 


and  therefore,  as  in  (2), 


£'«=--  L.R. 


(7) 


(8) 


Therefore  the  main  generator  supplies  approximately 
i/«th  the  unbalanced  load  current  at  full  voltage,  whereas 
the  local  mains  supply  this  load  current  at  E/nths  the  pres- 
sure between  outer  conductors. 


Fig.  180. 

In  the  case  of  a  4-wire  multivoltage  distribution  circuit, 
using  a  3-element  motor-generator  as  a  balancer,  with  cur- 
rents and  voltages  as  marked  and  directed  in  Fig.  180  the 
preceding  equations  apply  when  the  symbols  are  properly 
interpreted. 

Let  Et  equal  the  sum  of  the  voltages  generated  in  the 
armature  windings  of  all  the  elements  of  the  balancer,  and 
assume  that  the  different  armatures  generate  AEt,  BEt, 
and  CEi  volts  and  that  their  windings  have  resistances 
of  AR,  BR,  and  CR  ohms,  where  A+B+C  =  i.  Then 


2/8 


DYNAMO   ELECTRIC   MACHINERY. 


all  the  armatures  carry  the  no-load  current  /0,  and  each 
armature  carries  a  portion  of  the  unbalanced  load-current 
of  every  other  branch  than  the  one  to  which  it  is  directly 
connected,  as  well  as  a  portion  of  the  unbalanced  load-cur- 
rent of  the  branch  to  which  it  is  directly  connected.  The 
last  will  flow  in  a  direction  opposite  to  that  indicated  in  the 
figure.  According  to  the  foregoing  discussion,  (6)  and  (7)^ 
the  apportioning  is  as  follows  :  — 


and  7   =  / 


(A  -  i)I  A  + 
AIA  + 
AIA  + 


BIB  +  CIC, 

i)IB  +  CIC, 

BIB  +  (C  -  i)I  c  •      (9) 


The  pressures  between  successive  distribution  conductors 
are,  therefore, 

Eab  =  A(Et  +  7ttf), 

Ebc  =  B(Et  +  I2R), 
and  Ecd  =  C(Et  +  I3R).  (10) 


Fig.  181. 


The  equations  of  (9)  and  (10)  can  be  extended  to  embrace 
any  number  of  branches.  Balancers  having  three  elements, 
however,  satisfactorily  meet  the  requirements  of  multiple- 


BOOSTERS.  279 

voltage  systems  for  speed  control  of  motors.  Common 
branch  pressures  are  40,  80,  and  120  volts  respectively. 
By  use  of  such  a  system  a  very  wide  range  in  speed  is 
economically  possible.  A  four-wire  balancer  for  this  pur- 
pose is  shown  in  Fig.  181. 

112.  Boosters. — A  booster  is  a  machine  inserted  in 
series  in  a  circuit  to  change  its  voltage.  It  may  be  driven 
by  an  electric  motor,  in  which  case  it  is  termed  a  motor- 
booster^  or  otherwise. 

In  the  distribution  of  electric  energy  from  a  central  sta- 
tion, at  constant  potential,  it  often  happens  that  excessive 
currents  must  be  supplied  at  a  considerable  distance  from 
the  station  or  that  currents  of  ordinary  magnitude  must  be 
furnished  at  very  distant  points.  If  the  supply  potential  is 
to  be  the  same  at  the  distant  points  as  at  those  near  by,  and 
if  the  current  at  all  points  is  to  come  from  the  same  gen- 
erators, then  the  cross-section  of  a  feeder  to  a  distant  point 
needs  to  be  very  great,  unless  some  means  be  taken  to  com- 
pensate for  the  drop  in  pressure  caused  by  its  resistance. 
Series  boosters  are  frequently  employed  for  this  ^purpose. 
The  generator  element  of  the  booster,  in  such  a  case,  is 
connected,  at  the  station  or  at  any  other  point,  in  series 
with  the  feeder,  the  voltage-drop  in  which  is  to  be  compen- 
sated. As  the  drop  is  equal  to  the  current,  /,  carried  by 
the  feeder  times  its  resistance  Rf,  the  voltage  generated  in 
the  booster,  EB,  should  be  such  that 

—jr-  =  constant, 

a  condition  which  is  satisfied  by  a  generator  whose  charac- 
teristic is  a  straight  line,  Fig.  182,  making  an  angle  a  with 
the  abscissae  whose  tangent  is  EB/I.  Such  a  characteristic 


280  DYNAMO   ELECTRIC   MACHINERY. 

cannot  be  obtained,  but,  if  the  iron  of  the  magnetic  circuit 
be  not  fluxed  above  the  knee  of  the  magnetization  curve  by 
the  maximum  current  to  be  carried  by  the  feeder,  a  curved 
characteristic  may  be  obtained  which 
lies  sufficiently  close  to  the  straight 
line  to  yield  satisfactory  results. 
The  curve  lies  above  the  straight 
line  and  is  concave  towards  it  if  the 
compensation  is  perfect  at  full  load. 


o        LOAD  CURRENT         i      With  boosters  to  be  used  on  ordinary 
Flg* l82'  railroad  circuits,  the  maximum  vari- 

ation of  voltage  from  that  indicated  by  the  straight  line,  is 
generally  considered  to  be  permissible,  if  it  does  not  exceed 
10%  of  the  maximum  voltage  supplied  by  the  booster. 
As  the  field  flux  increases  with  the  load,  sparkless  commu- 
tation is  easily  obtained  at  full  load,  and  therefore  copper 
brushes  may  advantageously  be  employed  for  the  purpose 
of  increasing  the  efficiency. 

The  series  booster  is  also  frequently  used  in  electric 
railway  systems,  which  use  the  grounded  rails  for  returning 
the  propulsion  current  to  the  generating  station.  It  is  used 
to  reduce  the  portion  of  the  return  current  which  would 
otherwise  pass  through  the  earth  and  its  substructures.  It 
is  then  termed  a  negative  or  track-return  booster.  Consider 
a  point  on  the  track  rail  to  be  at  a  potential  above  the 
grounded  terminal  of  the  generator  at  the  station  and  that 
the  propulsion  current  is  returned  to  this  terminal,  under 
this  potential,  by  three  paths  connected  in  parallel,  namely 
the  track  rails,  the  earth,  and  a  negative  feeder  in  series 
with  a  negative  booster.  Representing  the  currents  flow- 
ing in  the  respective  paths  by  Ity  fe,  and  If,  and  the  resist- 
ances by  Rt,  Re,  and  R;,  then,  if  the  voltage  generated  by 


BOOSTERS.  28l 

the  booster  be  EB  and  if  it  be  properly  directed,  the  differ- 
ence of  potential  will  be 


"(*'-??) 


The  expression  in  parentheses  represents  the  apparent  resist- 
ance of  the  negative  feeder.  If  the  booster  be  so  designed 
that  the  slope  of  its  characteristic  is  approximately  Rf  the 
apparent  resistance  of  the  negative  feeder  becomes  zero, 
nearly  all  the  current  will  return  through  it,  and  the  earth 
currents  will  be  reduced. 

Sometimes  the  field  coil  of  the  negative  booster  is  con- 
nected in  series  with  the  outgoing  feeders  while  the  arma- 
ture is  connected  in  series  with  the  return  feeder  as  above. 

Boosters  are  extensively  used  in  connection  with  storage 
batteries  for  regulating  the  voltage  between  conductors  of 
constant-potential  systems. 

A  so-called  shunt  booster,  with  its  armature  connected,  in 
series  with  a  battery,  between  the  station  bus-bars,  and  with 
its  shunt  field  coils  also  connected  through  a  regulating 
rheostat  to  the  same  bus-bars,  is  often  used  to  increase  the 
voltage  impressed  upon  the  battery  above  that  between  bus- 
bars during  the  charging  of  the  battery.  Since  the  charg- 
ing pressure  per  cell  varies  from  1.8  volts  at  the  start  to 
2.65  volts  at  heavy  over-charge,  if  there  be  n  cells  and  E  volts 
between  bus-bars,  the  booster  will  be  required  to  furnish  at 
a  maximum  an  E.M.F. 

EB  =  ^(2.65  -  1.8)  =  0.85  n  volts, 

and  the  current  capacity  must  be  the  same  as  the  maximum 
charging  current  of  the  battery.  When  such  an  arrange- 
ment is  used  the  booster  is  employed  only  during  the 
charging  of  the  cells  and  not  all  the  cells  are  used  during 


282  DYNAMO   ELECTRIC   MACHINERY. 

the  whole  time  of  the  discharge  of  the  battery.  A  few 
cells,  called  end-cells,  are  cut  out  of  circuit  at  the  beginning 
of  the  discharge  and  are  successively  cut  in  again  as  the 
voltage  per  cell  decreases  due  to  the  discharge.  At  the 
close  of  the  discharge  all  the  cells  are  in  circuit  and  their 
number  is 

E 

n=T7s' 

The  E.M.F.  capacity  of  the  booster  as  well  as  its  normal 
volt-ampere  capacity  accordingly  amounts  to  about  40  % 
that  of  the  battery. 

Frequently   differentially-wound   boosters,   connected   as 
shown  in  Fig.  183,  are  used  in  connection  with  generators 


Fig.  183. 

supplying  energy  both  for  lighting  as  well  as  for  motors. 
This  arrangement  is  especially  suited  to  cases  where  the 
motor  load  fluctuates  much  while  its  average  value  is  small 
and  where  a  reduction  of  pressure  upon  the  motor  circuits 
under  load  is  advantageous.  Such  cases  are  found  in  office 
buildings,  apartment  houses,  and  hotels,  where  a  single  gen- 
erator supplies  energy  for  lamps  as  well  as  for  elevator  and 
pump  motors.  The  motor  bus-bars  generally  have  1 5  volts 
greater  potential  difference  than  the  lighting  bus-bars. 
At  average  motor  load,  the  shunt  ampere-turns  predomi- 
nate, the  series  coil  carries  the  average  motor-load  current> 
the  booster  E.M.F.,  EB>  is  added  to  the  generator  EM. P., 


BOOSTERS.  283 

and  the  battery  neither  charges  nor  discharges.  With 
heavy  motor  load,  the  series  excitation  falls  slightly  with  a 
consequent  fall  of  EB>  of  motor  bus-bar  pressure,  and  of 
shunt  excitation.  The  battery,  therefore,  supplies  the  ex- 
cess above  the  average  of  the  motor-load  current,  while  the 
main  generator  supplies,  as  before,  this  average.  On  light 
motor  load  EB  increases  slightly,  the  motor  bus-bar  pres- 
sure rises,  and  the  battery  takes  a  charging  current  equal 
to  the  difference  between  the  motor-load  current  and  its 
average  value.  The  current  in  the  booster  varies  in  prac- 
tice but  a  few  per  cent  from  the  average  motor-load  cur- 
rent, and  its  direction  is  always  the  same.  Such  a  machine 
is  therefore  called  a  non-reversible  or  a  constant-current 
booster. 

In  electric  railway  systems  where  there  is  a  large  aver- 
age current  the  battery  charge  and  discharge  rates  are 
moderate  and  it  is  desirable  that  the  voltage  should  not 
decrease  with  increase  of  load.  In  such  cases  the  differ- 
entially-wound booster  may  be  employed,  with  connections 
as  indicated  in  Fig.  1 84,  where  the  direction  of  the  current 


MOTOR 
LOAD 


Fig.  184. 


through  the  armature  alters  with  change  from  charge  to  dis- 
charge of  the  battery.  It  is  therefore  called  a  reversible 
booster.  At  normal  load,  the  series  and  the  shunt  ampere- 
turns  are  equal  and  opposed  to  each  other,  and  hence  the 


284 


DYNAMO   ELECTRIC   MACHINERY. 


booster  E.M.F.,  EB)  is  zero,  the  battery  is  neither  charging 
nor  discharging  and  its  open-circuit  voltage,  Es,  is  equal  to 
£,  that  of  the  generator  and  system.  With  heavy  loads  the 
series  ampere-turns  predominate,  EB  is  added  to  E&  and  the 
battery  discharges.  With  light  loads  the  shunt  ampere- 
turns  predominate,  EB  opposes  Es,  and  the  battery  charges. 
When  the  battery  is  discharged  its  E.M.F.  falls,  but  the 
booster  compensates  therefor  by  taking  more  current  in 
the  series  coil.  The  load  on  the  generator  is  practically 
constant  and  the  battery  takes  up  the  variations.  The 
booster  has  to  carry  the  maximum  battery  current  and  it 
must  at  the  same  time  give  its  maximum  E.M.F. ;  and 
these  values,  therefore,  determine  its  capacity. 

The  series  coils  of  the  two  differentially-wound  boosters 
must  be  of  sufficient  cross-section  to  carry  very  large  cur- 
rents, and  this  again  requires  such  large  magnet  frames  as 
to  make  the  cost  of  the  machines  excessive.  Means  have 
therefore  been  devised  for  making  use  of  shunt-wound 
boosters,  the  current  in  the  shunt  coil  being  changed  by 
and  in  accordance  with  variations  in  the  generator  current. 


Fig.  185. 

The  Hubbard  booster  system  makes  use  of  an  auxiliary 
generator,  X,  for  furnishing  exciting  current  for  the  booster, 
the  connections  being  as  indicated  in  Fig.  185.  In  prac- 
tice the  exciter  and  driving  motor  are  on  one  shaft.  At 
average  load  the  voltage  of  the  exciter,  Ex)  is  the  same  as 


BOOSTERS. 


285 


that  of  the  system,  E,  and  opposed  to  it,  the  booster  volt- 
age, EB,  is  zero,  and  the  battery  neither  charges  nor  dis- 
charges. With  heavy  load  Ex  >  E,  EB  is  added  to  Es,  and 
the  battery  discharges.  With  light  load,  EX<E,  EB  is  re- 
versed and  is  opposed  to  Es,  and  the  battery  charges. 
This  system  is  sometimes  called  the  counter  E.M.F.  system 
because  the  exciter  E.M.F.  opposes  that  of  the  system. 

Another  much-used  booster  system  employs  the  Entz 
carbon-plate  regulator  to  control  the  exciting  current  in  an 
auxiliary  generator  used  as  an  exciter  for  the  booster.  The 


Fig.  186. 

connections  are  as  indicated  in  Fig.  186.  Two  piles  of 
carbon  plates  /  and  r  have  variable  resistances,  which  are 
reduced  under  pressure,  and  whose  magnitudes  are  con- 
trolled by  the  combined  action  of  the  solenoid  5  and  the 
spring  s.  The  resistance  Rt  is  reduced  and  Rr  increased 
upon  increase  of  current  in  S.  At  average  load  Rl 
and  Rr  are  equal  and  therefore  the  point  a  is  at  the  same 
potential  as  b,  and  there  is  no  current  in  the  field  coil  of 
the  exciter  X.  Hence  Ex  —  EB  =  o,  Es  =  E,  and  the  battery 
neither  charges  nor  discharges.  With  heavy  load  R[<Rr, 
Ex>o,  EB  has  the  same  direction  as  Es,  and  the  battery 
discharges.  With  light  load  Rr  <  Rh  Ex  >  o,  EB  is  opposed  to 


286 


DYNAMO   ELECTRIC   MACHINERY. 


Es,  and  the  battery  charges.     The  appearance  of  the  regu- 
lator is  shown  in  Fig.   187. 

Booster  armatures  are  generally  lap-wound  because  of 
the  large  currents  which  they  must  carry.  The  current 
density  in  the  windings  can  be  made  high  because  they  are 


Fig.  187. 

rarely  called  upon  to  give  their  rated  output.  The  reversible 
boosters  should  have  laminated  field-magnet  cores  in  order 
to  avoid  a  sluggish  behavior.  Sparking  is  liable  to  occur, 
unless  the  armature  coils  are  of  low  reactance,  because  of 
the  weak  fluxing  of  the  iron  of  the  magnetic  circuit.  Safety 
relay  devices  should  be  used  to  prevent  racing  of  the  gen- 


STORAGE   BATTERIES.  287 

erator  element  in  case  of  the  accidental  opening  of  the  shunt 
exciting  circuit  of  the  motor  element. 

113.  Storage  Batteries.  —  Storage  batteries  are  reversible 
electrolytic  cells,  whose  electrodes  are  chemically  modified 
by  the  passage  of  current  through  the  cells  and  which 
thereby  absorb  and  store  energy  when  the  current  flows  in 
one  direction  and  give  it  up  when  the  direction  is  reversed. 
The  commercial  forms  make  use  of  lead  for  electrodes  and 
dilute  sulphuric  acid  for  an  electrolyte.  When  charged,  and 
energy  has  been  absorbed,  the  positive  electrode,  that  is  the 
one  of  higher  potential,  is  modified  so  as  to  contain  an 
amount  of  lead  peroxide  (PbO2),  while  the  other  or  neg- 
ative electrode  contains  a  corresponding  amount  of  sponge 
lead. 

Before  a  commercial  cell  can  be  considered  as  ready  to 
receive  its  first  or  factory  charge,  the  electrodes  must  have 
been  materially  modified  from  the  condition  of  ordinary  reg- 
uline  lead.  Their  surfaces  may  have  been  rendered  porous 
by  mechanical  and  electrochemical  treatment,  or  lead  oxides 
may  have  been  conductively  united  with  them  through  me- 
chanical means.  The  plates  constituting  the  former  elec- 
trodes are  termed  Plante plates,  while  the  latter  are  known 
as  pasted  plates.  The  purpose  of  the  preliminary  treatment 
or  formation  of  the  electrodes  is  eventually  to  expose  a  large 
surface  to  the  electrolyte,  so  that,  under  the  limitations  as 
to  the  velocities  of  the  chemical  reactions,  a  relatively  high 
rate  and  large  amount  of  energy  may  be  absorbed  during 
charge  and  be  liberated  during  discharge.  When  fully 
charged,  the  porous  portion  of  the  positive  electrode  is 
peroxide  of  lead  and  of  the  negative  is  sponge  lead.  At  all 
other  times  there  is  some  lead  sulphate  present  in  the 
porous  or  active  material  of  both  electrodes.  As  lead  sul- 


288  DYNAMO   ELECTRIC   MACHINERY. 

phate  is  a  non-conductor  of  electricity,  an  excessive  amount 
of  it  will  interfere  with  the  functioning  of  the  active  mate- 
rial. When  the  open-circuit  potential  of  a  cell  sinks  to  1.8 
volts  the  amount  has  increased  to  the  permissible  limit. 
The  chemical  changes  taking  place  during  charge  and  dis- 
charge are  represented  by  the  formula 

Charge 

Pb02  +  Pb  +  2  H,S04  =  2  PbS04  +  2  H20. 

Discharge 

Plante  plates  are  larger,  heavier,  more  expensive,  and 
more  likely  to  be  injured  by  impurities  in  the  electrolyte 
than  pasted  plates,  although  they  are  more  efficient,  dura- 
ble, and  dependable.  They  are  best  fitted  for  use  in  con- 
nection with  central  stations.  Pasted  plates  are  to  be  pre- 
ferred for  motor-car  propulsion.  The  electrochemical  action 
does  not  penetrate  much  more  than  a  millimeter  below  the 
surface  of  the  electrode,  because  the  active  material  has  so 
much  greater  conductivity  than  the  electrolyte  and,  at  that 
depth,  most  of  the  current  is  confined  to  the  active  ma- 
terial, and  there  is,  accordingly,  no  appreciable  release  of 
ions  from  the  electrolyte. 

The  acid  of  the  electrolyte  should  be  made  from  sulphur 
and  not  from  pyrites  and  at  full  charge  should  be  diluted 
to  have  a  specific  gravity  of  1.20.  During  discharge  the 
electrolyte  gives  up  to  the  electrodes  SO3  and  its  specific 
gravity  therefore  falls  from  1.13  to  1.19  depending  upon 
the  amount  of  electrolyte.  Calculations  of  the  resistance 
offered  by  the  electrolyte  can  be  based  upon  the  assump- 
tion that  its  resistivity  is  4/3  ohm  per  cubic  centimeter  at 
1 8°  C.  with  a  negative  temperature  coefficient  of  0.016  per 
degree  Centigrade. 


STORAGE    BATTERIES. 


289 


The  E.M.F.  of  a  cell  depends  upon  the  concentration  of 
the  electrolyte  and  varies  in  accordance  with  the  curve 
shown  in  Fig.  188.  It  also  depends  upon  the  condition  of 


2.0 


51.8 
1.7 
1.6 


1.5 


LOG 


1.10  1,15 

SPECIFIC   GRAVITY 

Fig.   188. 


1.2 


1.25 


charge.     The  terminal  voltages  during  the  hours  of  charge 
and  discharge  as  a  function  of  the  time  are  shown  in  Fig.  1 89. 


2.6 
2.4 

2.2 
> 
i 

'2.00 
1.8 
1.6 


34 

HOURS 


Fig.  189. 


The  E.M.F.  of  a  fully  charged  cell  is  2.5  volts.  If  an 
auxiliary  cadmium  electrode  be  inserted  in  the  electrolyte 
its  potential  should  be  2.3  volts  below  that  of  the  positive 
plate  and  0.2  volt  above  that  of  the  negative  plate.  When 


290  DYNAMO   ELECTRIC    MACHINERY. 

fully  discharged  the  E.M.F.  of  a  cell  is  1.8  volts  and  cad- 
mium should  have  a  potential  2.05  below  the  positive  elec- 
trode and  0.25  below  the  negative.  By  means  of  such  a 
cadmium  test  the  condition  of  either  electrode  can  be  deter- 
mined. It  is  common  to  take  the  average  E.M.F.  of  a  cell 
as  2  volts.  To  obtain  a  battery  of  greater  E.M.F.  a  plural- 
ity of  cells,  connected  in  series,  is  employed. 

The  capacity  rating  of  a  storage  cell  is  expressed  by  the 
number  of  ampere-hours  which  it  will  furnish  in  discharging 
itself  at  constant  current  from  a  fully  charged  condition  to 
a  point  where  its  potential  on  open  circuit  is  1.8  volts,  the 
discharge  being  completed  after  the  expiration  of  8  hours. 
The  actual  ampere-hour  capacity  decreases  with  an  increase 
of  discharge  rate,  that  is  if  made  in  less  than  8  hours.  It  is 
reduced  to  one-half  if  made  in  one  hour.  The  capacity  is 
from  40  to  60  ampere-hours  per  square  foot  of  exposed 
area  of  positive  electrode,  counting  both  sides  but  taking 
no  account  of  increase  of  surface  due  to  porosity.  The 
normal  current  rate  of  charge  or  discharge  is  therefore  from 
5  to  8  amperes  per  square  foot.  A  continuous  discharge 
should  not  exceed  25  amperes  per  square  foot.  Double  this 
rate  is  permissible  for  30  seconds  or  less.  If  it  be  desired  to 
charge  the  cell  rapidly  the  charging  current  should  not  be 
kept  constant.  To  charge  in  three  hours,  for  example,  the 
current  for  the  first  hour  should  be  4  times  the  8-hour  rate, 
for  the  next  2. 5  times,  and  for  the  last  1.5  times.  Theoretic- 
ally the  amount  of  lead  chemically  modified  per  ampere-hour 
on  either  electrode  is  0.135  oz.  Practically  from  0.5  to  0.9 
oz.  is  required.  There  should  be  at  least  TV  Ib.  of  elec- 
trolyte for  each  ampere-hour. 

It  is  customary  to  connect  numbers  of  positive  plates 
by  a  lead  lug  to  form  the  positive  electrode  of  a  cell  and 


STORAGE    BATTERIES. 


291 


to  use  one  or  more  negative  plates  for  the  negative  electrode. 
These  are  assembled  in  the  electrolyte  so  that  successive 
plates  are  of  opposite  polarity  and  both  sides  of  each  posi- 
tive plate  are  exposed  to  a  negative.  Containing  jars  are 


Fig.  190. 

made  of  glass  or  hard  rubber,  and  for  large  cells  lead-lined 
wooden  tanks  are  used.  Fig.  190  shows  a  56o-ampere-hour 
cell  in  a  glass  jar.  The  power  output  per  pound  of  complete 
cell  is  from  8  to  14  watts  with  pasted  plates  and  from  3  to  7 
watts  with  Plante  plates. 


2Q2  DYNAMO   ELECTRIC    MACHINERY. 

PROBLEMS. 

1.  A  dynamotor,  the  resistances  of  whose  armature  windings 
are  each  o.  i  ohm,  is  used  as  an  equalizer  on  a  3-wire  equivolt- 
age  system  with  200  volts  between  outside  conductors.     One 
ampere  flows  through   the  armatures  when  the  system  is  bal- 
anced.    Find  the  power  expended  in  frictions  and  the  counter 
E.M.F.  of  each  armature  winding. 

2.  Find  the  regulation  of  each  side  of  the  system  of  the  pre- 
ceding problem  if  the  maximum  unbalanced  load  be   100  am- 
peres. 

3.  A  motor-generator,  with  one  armature  having  a  resistance 
two-thirds  as  great  as  the  other  and  designed  to  generate  at 
the  same  speed  two-thirds  the  E.M.F.  generated  by  the  other, 
is  used  as  a  balancer  between  outer  wires  having  a  potential  dif- 
ference of  500  volts.     The  power  required  to  overcome  frictions 
is  400  watts  and  the  resistance  of  the  two  armatures  in  series  is 
1.2  ohms.     A  current  of  150  amperes  flows  through  the  middle 
wire.     Determine  the  capacity  of  each  unit  of  the  balancer. 

4.  A  3-element  four-wire  balancer  is  used  on  a  multivoltage 
variable-speed  motor  system  with  40,  120,  and  80  volts  between 
successive  wires.     The  armature  resistances  are  proportional  to 
the  E.M.F.  generated  in  them,  and  together  amount  to  0.48 
ohm.     On  balanced  load  the  armatures  take  10  amperes.     If  the 
current  between  successive  line  wires  be3o,75,andio  amperes 
respectively,  what   are  the  respective  currents  in  the  armatures 
of  the  motor-generator  elements  ?     What  is  the  pressure  between 
successive  wires  ? 

5.  From  a  point  one  mile  distant  there  returns  to  the  generat- 
ing station  for  a  single-track  railway,  that  uses  yo-lb.  (per  yard) 
rails,  200  amperes  by  way  of  the  rails.     Two  parallel  No.  oooo 
copper  wires  in  series  with  a  negative  booster  and  returning 
300  amperes  to  the  station,  will  reduce  to  zero  the  drop  between 
this  point  and  the  station  with  what  booster  voltage  ?     The  re- 
sistivity of  the  rails,  including  bonds,  is  no  ohms  per  mil-foot. 


PROBLEMS. 


293 


6.  In  Fig.  191  the  curve  represents  a  maximum  daily 
load-curve,  on  a  5oo-volt  system,  variations  in  which  are  to  be 
provided  for  by  a  storage  battery.  Determine  the  current  /av 
such  that  the  charging  ampere-hours  shall  exceed  the  quantity 
of  discharge  by  10%,  and  then  obtain  the  ampere-hour  capacity 
and  the  exposed  area  of  positive  plates  per  cell  and  the  number 
of  cells  in  an  appropriate  battery  having  end-cell  regulation. 


700 


600 


500 


o: 

UJ400 


Q300 


200 


100 


12     2     4     6     8     10    12     2     4     6     8     10    12 

HOURS 

Fig.  191. 

7.  At  500  volts  with  battery  arranged  as  in  Fig.  184,  how 
many  cells  would  be  required  in  the  battery  ? 

8.  Plot  a  power  output-time  curve  of  the  reversible  booster  of 
the  preceding  problem,  the  load  curve  being  that  shown  in  Fig. 
191;  each  cell  having  an  ampere-hour  capacity  as  in  problem 
6,  the  voltage-hour  curves  of  charge  and  discharge  being  those 
shown  in  Fig.  189,  and  the  cell  being  considered  as  fully  charged 
at  the  instant  when  the  load  first  assumes  its  average  value. 

9.  If  the  booster  and  its  driving  motor  each  have  an  overload 
capacity  of  25%  for  two  hours  and  have  an  efficiency  of  90% 
at  full  load,  what  is  the  capacity  of  each  in  kilowatts  ?    What  is 
the  maximum  motor  input  ? 


294  DYNAMO   ELECTRIC   MACHINERY. 


CHAPTER    IX. 
CENTRAL-STATION    EQUIPMENT. 

1 14.  Paralleling  of  Generators.  —  In  general  a  generator 
is  much  more  efficient  when  operated  at  its  full  load  than 
when  operated  at  one-half  or  one-quarter  load.  It  is  usual 
to  install  in  central  stations,  which,  as  a  rule,  must  supply 
different  quantities  of  electrical  energy  at  different  times  of 
the  day,  a  number  of  smaller  units  rather  than  one  unit 
large  enough  to  supply  the  total  energy.  By  this  means 
any  load  can  be  handled  by  a  machine  or  by  a  number  of 
machines  all  operating  at  about  their  maximum  efficiency. 
It  is  necessary,  therefore,  to  consider  the  methods  of  com- 
bining two  or  more  machines  to  supply  energy  to  a  single 
load. 

The  simplest  and  most  usual  method  of  connecting 
generators  is  that  employed  in  incandescent  light  generat- 
ing stations,  where  a  number  of  constant-pressure  machines 
are  connected  in  parallel,  the  positive  and  negative  terminals 
of  each  generator  being  connected  respectively  to  positive 
and  negative  common  conductors,  called  bus-bars,  which  are 
located  on  the  rear  of  the  operating  switchboard.  The 
connections  of  two  shunt-wound  machines  with  their  regu- 
lating rheostats  are  shown  in  Fig.  192.  The  various  load 
external  circuits  are  connected  in  parallel  to  the  bus-bars. 
This  practice  is  frequently  modified  by  separating  those 
machines  which  supply  energy  to  the  more  distant  loads 
from  those  that  supply  the  shorter  circuits.  This  is  because 


CENTRAL-STATION   EQUIPMENT. 


295 


the  maintaining  of  a  constant  and  uniform  pressure  at  all 
distributing  points  requires  a  higher  pressure  on  the  station 
ends  of  the  longer  mains  than  that  on  the  shorter.  When 
a  machine  is  to  be  connected  to  bus-bars  to  which  other 
operating  machines  are  already  connected,  it  is  first  brought 


+  BUS 


\ 


-pmjL 


Fig.  192. 

up  to  speed  ;  the  field  magnetization  is  then  adjusted  till 
the  machine  generates  the  same  voltage  as  that  of  the  bus- 
bars, and  the  main  switch  is  then  closed,  which  puts  the 
machine  in  circuit.  The  proper  voltage  at  which  to  connect 
in  the  new  machine  may  be  roughly  determined  by  compar- 
ing the  relative  brightness  of  its  pilot  lamp  with  that  of  the 
lamps  operating  on  the  circuit.  A  more  exact  way  is  to 
compare  the  readings  of  a  voltmeter  across  the  terminals  of 
a  machine  with  one  across  the  bus-bars.  Another  method 
is  to  connect  the  generator  to  the  bus-bars  through  a  high 
resistance  and  a  galvanometer  indicator.  When  the  latter 
indicates  no  deflection,  the  voltages  of  the  machine  and  the 
bus-bars  are  identical,  and  the  machine  may  be  connected 
in.  Sometimes  the  differential  indicator  is  used  for  this 
purpose. 

When  shunt  machines  are  connected  in  parallel,  their  volt- 
ages should  be  maintained  the  same  so  that  the  total  load 


296 


DYNAMO   ELECTRIC    MACHINERY. 


may  be  properly  apportioned  among  them.  If  this  equality 
of  voltage  be  not  maintained,  no  serious  damage  will  occur, 
oince  the  machine  which  generates  the  lower  voltage  merely 
fails  to  take  its  full  share  of  the  load.  Even  if  the  voltage 
of  one  machine  falls  so  low  that  it  is  overpowered  and  run 
as  a  motor,  still  no  damage  will  result,  save  perhaps  the 
blowing  of  a  fuse,  since  the  direction  of  rotation  for  a  shunt 
machine  is  the  same  whether  it  be  run  as  a  generator  or  as 
a  motor.  If  it  be  desired  to  regulate  a  number  of  machines 
simultaneously  by  one  regulator,  it  may  be  accomplished  by 
bringing  the  positive  ends  of  the  field  coils  to  one  terminal 
of  the  regulator  and  connecting  the  other  terminal  to  the 
negative  bus. 

Shunt  machines  may  be  operated  in  series  by  connecting 
the  positive  brush  of  one  machine  to  the  negative  brush  of 

the   next,    and    connecting   the 
extreme    outside    brushes    with 
the  line  wires.     Each  machine 
can  be  regulated  separately  to 
generate    any    portion    of    the 
pressure,  or,  if  it  be  desired  to 
._      regulate  all  the  machines  thus 
Fis-  J93-  connected    uniformly  and  as  a 

unit,  the  field  coils  of  all  the  machines  may  be  joined 
in  series  with  one  regulating  rheostat,  and  shunted  across 
the  line  wires.  Fig.  193  illustrates  such  an  arrangement 
for  two  1 1 5 -volt  generators. 

Series-wound  generators  may  be  operated  in  series,  as  in 
the  Thury  system  of  direct-current  high-potential  power 
transmission,  which  is  in  use  on  a  number  of  lines  in 
Europe.  The  aggregate  capacity  of  the  1 5  plants  at  pres- 
ent employing  this  system  is  25,000  horse-power,  the  line 


CENTRAL-STATION   EQUIPMENT.  297 

from  Montiers  to  Lyons  being  the  longest  (112  miles)  and 
employing  the  highest  voltage  (57,000  volts). 

There  are  a  number  of  groups  of  generators  at  the  power 
house  connected  in  series,  each  group  being  driven  by  a 
water  turbine  or  other  prime  mover ;  the  voltage  per 
machine  being  less  than  4000.  Each  generator  is  insulated 
from  ground  and  from  the  other  machines  of  the  group, 
the  middle  point  of  the  system  being  grounded  to  limit  the 
required  insulation.  The  line  current  is  maintained  con- 
stant by  several  complicated  auxiliary  devices.  These  auto- 
matically regulate  the  speed  of  all  the  prime  movers,  so  as 
to  keep  the  line  voltage  proportional  to  the  load;  cut  in 
or  out  of  circuit  one  or  more  machines  if  there  be  large 
changes  in  load ;  and  short-circuit  any  disabled  machine.  In 
the  substations,  a  number  of  series-wound  motors  are  con- 
nected in  series  across  the  line,  the  motors  being  arranged 
in  groups,  each  group  driving  a  generator.  The  generators, 
which  may  deliver  either  direct  or  alternating  current,  are 
connected  in  multiple  for  distribution.  The  motors  and 
generators  in  the  substations  are  insulated  from  each  other 
and  from  ground,  just  as  are  the  machines  in  the  generating 
station.  The  current  taken  by  the  motors  is  kept  constant 
by  an  automatic  shifting  of  the  brushes.  The  Thury  sys- 
tem is  adapted  only  to  undertakings  where  the  power  is  to 
be  transmitted  over  a  long  distance  and  the  load  is  to  be 
concentrated  at  few  points,  since  at  every  tap  a  complete 
substation  must  be  provided  containing  motors  having  an 
aggregate  voltage  equal  to  the  line  voltage.  If  the  line 
alone  be  considered,  direct-current  power  transmission  is  far 
superior  to  alternating-current  transmission,  which  is  em- 
ployed exclusively  in  this  country.  Less  conductor  mate- 
rial is  required  and  none  of  the  disturbing  influences  met 


298 


DYNAMO   ELECTRIC    MACHINERY. 


with  in  alternating-current  transmission  are  encountered  in 
the  Thury  system. 

Difficulty  is  experienced  if  it  be  attempted  to  operate 
series  generators  in  parallel.  If  the  machines  start  with  a 
proper  distribution  of  load  among  them,  and  if  one  generates 
slightly  less  than  its  full  voltage,  then  this  machine  does 
not  continue  to  take  its  full  share  of  the  load;  and,  since  it 
is  series  wound,  the  magnetic  field  becomes  weakened,  thus 
resulting  in  a  still  lower  voltage.  The  conditions  continually 
grow  more  uneven  until  the  machine  is  overpowered  and 
it  becomes  a  motor.  Since  the  direction  of  rotation  of  a 
series-wound  motor  is  opposite  to  its  direction  when  run  as 
a  generator,  serious  results  may  occur.  One  way  in  which 
this  difficulty  may  be  overcome  is  to  arrange  the  field  coils 
so  that  the  magnetization  in  any  one  machine  will  remain 
the  same  as  in  the  other  machines,  even  though  its  pressure 
falls  below  that  of  the  others.  To  accomplish  this  result 
the  series  fields  must  all  be  placed  in  parallel.  This  may 
be  done  by  means  of  an  equalizer,  which  is  a  wire  of  small 
resistance  connecting  all  of  the  brushes  of  one  polarity,  and 


Fig.  194. 


placing  the  field  in  parallel,  as  shown  in  Fig.  194.  Two 
series  dynamos  may  be  run  in  parallel  without  an  equalizer 
by  resorting  to  mutual  excitation,  that  is,  by  letting  the  cur- 


CENTRAL-STATION   EQUIPMENT. 


299 


rent  of  one  armature  excite  the  field  of  the  other.  In  this 
case,  if  the  pressure  of  one  machine  falls  and  its  load  there- 
fore decreases,  the  magnetization  of  the  other  is  reduced, 
compelling  the  first  to  maintain  its  share  of  the  load. 
Series  dynamos  are  never  operated  in  parallel  in  prac- 
tice, but  this  discussion  is  introduced  because  of  its 
application  to  the  parallel  operation  of  compound-wound 
dynamos. 

Compound-wound  generators  are  extensively  used  for 
constant-potential  distribution.  Since  these  machines  have 
series  field  coils  as  well  as  shunt  field  coils,  the  parallel 
connection  thereof  for  combined  output  should  involve  an 
equalizing  bus,  as  in  Fig.  195.  Any  number  of  compound 
generators  may  be  op- 


I  MAINS 


EQUALIZER 


FIELD   COILS 


Fig-  195- 


erated  in  parallel  regard- 
less of  the  size  of  the 
units,  provided  their  volt- 
ages are  the  same  and 
that  the  resistances  of 
their  series  field  coils  are 
inversely  proportional  to 
the  currents  supplied  by  the  individual  machines.  Over- 
compounded  generators  which  are  to  be  operated  in  parallel 
must  yield  exactly  the  same  voltage  increase  at  full  load 
or  at  any  other  load.  This  may  be  adjusted  by  interposing 
additional  resistance  in  the  series  field  coil  of  that  generator 
which  carries  more  than  its  share  of  the  load. 

115.  Parallel  Operation  of  Motors. — Any  number  of 
shunt  motors  may  be  placed  in  parallel  across  constant- 
pressure  mains,  and  their  operation  will  be  satisfactory 
whether  each  has  a  separate  load  or  whether  they  be  con- 
nected through  suitable  devices  to  a  common  shaft.  Shunt 


300  DYNAMO   ELECTRIC   MACHINERY. 

motors  will  operate  in  series  on  a  constant-pressure  circuit 
when  positively  coupled  together;  but  if  connected  to  the 
same  shaft  by  belts,  and  one  belt  slips  or  comes  off,  that 
motor  will  race,  and  receive  more  than  its  proper  portion 
of  the  voltage.  This  arrangement  is  not  common. 

Series  motors  will  operate  satisfactorily  on  constant- 
pressure  circuits  if  rigidly  coupled  to  their  loads.  Series 
motors  connected  in  series  on  constant-pressure  mains  will 
operate  satisfactorily,  dividing  up  the  total  voltage  between 
them  according  to  the  load  each  is  carrying.  If  it  be 
desired  to  make  them  share  a  load  equally  they  must  be 
geared  together  so  that  each  rotates  at  the  speed  correspond- 
ing to  its  share  of-  the  voltage.  Series  motors  only  are  used 
on  constant-current  circuits.  Any  number  of  these  may  be 
placed  in  series  on  such  a  circuit  individually  or  connected 
to  a  common  shaft.  A  series  motor  on  a  constant-current 
circuit  may  be  overloaded  until  it  stops  without  harm,  since 
a  constant  current  flows  at  any  speed. 

Compound-wound  motors  are  coming  into  quite  general 
use,  and  they  are  invariably  operated  on  constant-pressure 
circuits,  and  each  machine  has  its  own  load. 

116.  Switches.  —  Switches  are  devices  inserted  in  a  cir- 
cuit to  facilitate  its  establishment  or  interruption.  They 
are  generally  of  the  form  known  as  knife  switches,  and  may 
be  single-pole,  double-pole,  etc.,  according  to  the  number 
of  circuit  interruptions  simultaneously  effected.  Fig.  196 
shows  a  3OOO-ampere  125-volt  double-throw  back-connected 
multiple-blade  knife  switch  made  by  the  Anderson  Manu- 
facturing Company. 

The  metal  parts  of  such  switches  consist  of  copper  hinges, 
blades,  and  clips,  and  these  must  be  properly  designed  to 
have  sufficient  current-carrying  capacity  and  contact  surface. 


CENTRAL-STATION    EQUIPMENT. 


301 


It  is  usual  to  allow  one  square  inch  of  cross-sectional  area 
for  every  800  to  1000  amperes,  and  to  provide  one  square 
inch  of  contact  surface  for  every  60  to  75  amperes  of  cur- 
rent. The  distances  between  metal  parts  of  opposite 
polarity  and  the  break  distances  of  approved  switches,  in 


Fig.  196. 


inches,  are  given  in  the  following  table.  Frequently 
switches  are  provided  with  fuse  connections,  and  also  lugs 
into  which  the  ends  of  the  leads  are  soldered. 


MINIMUM    SEPARATION 

SIZE   OF 

OPPOSITE   POLARITY 

SWITCH 

125  v. 

125  v. 

250  v. 

125  v. 

125  v. 

250  v. 

OR 

TO 

TO 

OR 

TO 

TO 

LESS 

250  v. 

600  v. 

LESS 

250  v. 

600  v. 

ic  amp.  or  less 

I 

Ij£ 

3K 

% 

M 

3 

10-35  amperes 

*/€ 

iM 

4 

I 

i% 

3/2 

35-100       « 

*X 

2/€ 

4/^ 

1% 

2 

4 

100-300 

26 

z% 

2 

2% 

300-600      " 

2% 

2% 

2% 

2/^ 

600—1000    " 

3 

3 

2% 

2% 

302 


DYNAMO    ELECTRIC   MACHINERY 


\ 


Fig.  197- 

A  10000  ampere  500  volt  " rotary"  switch  for  use  on 
central-station  switchboards  is  shown  in  Fig.  197. 

117.  Fuses.  —  Fuses  are  devices  intended  to  protect  cir- 
cuits from  destruction  or  damage  which  might  result  from  an 
excessive  flow  of  current  through  them.  They  are  made 


CENTRAL-STATION   EQUIPMENT.  303 

of  fusible  material,  generally  of  lead  or  an  alloy  of  tin  and 
lead,  and  take  the  form  of  wire  or  strips  provided  at  each 
end  with  a  copper  terminal  which  is  slotted  to  fit  into  fuse 
receptacles. 

The  magnitude  of  the  current  which  will  melt  a  fuse  de- 
pends upon  the  length  of  the  wire.  Short  lengths  of  a  wire 
of  given  cross-section  and  given  material  will  carry  larger 
currents  than  longer  lengths.  The  heat  which  is  generated 
in  the  short  ones  escapes  more  rapidly,  owing  to  the  prox- 
imity of  large  masses  of  metal  which  commonly  form  the 
terminals  of  the  fuse.  Fuses  are  rated  at  80  per  cent  of 
the  greatest  current  they  can  carry  indefinitely  without 
melting.  This  rating  enables  the  fuse  to  carry  a  current 
25  per  cent  greater  than  the  normal  current  for  which  the 
fuse  is  designed. 

The  " blowing"  of  a  fuse  is  accompanied  by  a  flash  and 
a  spattering  of  the  fused  metal ;  this  may  ignite  near-by 
combustible  materials.  Therefore  link  fuses  should  be 
placed  in  separate  porcelain  or  other  fireproof  receptacles. 
The  better  practice  is  to  use  enclosed  fuses,  in  winch  the 
fusible  conductor  is  surrounded  by  a  finely  divided  powder 
contained  in  an  insulating  casing. 

1 1 8.  Circuit  Breakers. — In  central-station  practice  the 
fuse  with  its  uncertainties  has  been  superseded  by  the 
electro-magnetic  circuit  breaker.  This  device  acts  promptly 
and  definitely,  and  has  the  advantage  that  a  circuit  once 
opened  by  it,  due  to  an  excessive  current  therein,  may  be 
instantly  reestablished. 

A  circuit  breaker  consists  of  a  switch  which  may  be 
closed  against  the  action  of  a  strong  spring  and  kept  closed 
by  means  of  a  latch.  This  latch  is  controlled  by  the 
plunger  of  a  solenoid  which  is  connected  in  series  with  the 


304 


DYNAMO   ELECTRIC    MACHINERY. 


line.  When  an  abnormal  current  flows  through  the  circuit, 
the  plunger  is  attracted  and  strikes  against  a  trigger  which 
releases  the  latch.  The  spring  then  becomes  operative  and 
the  circuit  breaker  is  opened.  The  opening  of  a  circuit 
carrying  an  excessive  current  is  accompanied  by  an  arc 
across  the  break.  To  avoid  arcing  across  the  metal  con- 
tacts of  a  circuit  breaker,  this  device  is  provided  with  an 
auxiliary  set  of  carbon  contacts  so  arranged  that  the  latter 
are  opened  an  instant  later  than  the  main  metal  contacts. 
Thus  all  the  sparking  takes  place  at  the  renewable  carbon 
contacts. 

A  Westinghouse  single-pole  5OO-volt  circuit  breaker  for 
use  with  compound-wound  generators  is  shown  in  Fig.  198. 

The  solenoid  is  a  massive 
copper  coil  located  on  the 
rear  of  the  marble  panel. 
The  movable  member  is  built 
up  of  thin  sheets  of  spring 
copper  which  contact  edge- 
wise against  solid  copper  ter- 
minal blocks.  This  instru- 
ment may  be  adjusted  to 
open  the  circuit  which  it  pro- 
tects for  any  predetermined 
current  strength  between  the 
limits  of  20  %  less  than,  and 
50  %  in  excess  of,  the  normal 
Fig.  198.  current. 

When  generators  are  operated  in  parallel,  each  machine 
should  be  protected  by  a  reverse-current  circuit  breaker, 
which  will  open  when  a  reverse  current  of  predetermined 
value  flows.  The  usual  overload  circuit  breaker,  just  de- 


CENTRAL-STATION   EQUIPMENT.  305 

scribed,  may  be  used  for  this  purpose  when  provided  with 
a  relay  ;  or,  better  still,  a  circuit  breaker  equipped  with  two 
solenoids  may  be  used,  one  of  which  operates  on  the  passage 
of  an  excessive  current. and  the  other  on  the  flowing  of  a 
reverse  current.  A  circuit  breaker  especially  suitable  for 
storage-battery  work  to  prevent  the  current  from  flowing 
back  through  the  generator  is  the  underload  circuit  breaker. 
It  is  usually  adjusted  to  open  the  circuit  when  the  current 
falls  below  one-tenth  of  its  normal  rated  value.  Another 
type  of  circuit  breaker  is  that  having  a  no-voltage  release  ; 
this  instrument  is  particularly  adapted  for  the  protection 
of  motor  circuits  from  dangers  accruing  from  the  circuit 
remaining  closed  while  the  line  is  idle. 

119.  Measuring  Instruments.  —  The  instruments  em- 
ployed in  direct-current  work  are  ammeters,  voltmeters  and 
wattmeters,  for  measuring  respectively  current,  voltage  and 
power.  Every  instrument  has  some  movable  part  to  which 
a  pointer,  passing  over  a  divided  scale,  is  attached.  Two 
forces  act  upon  this  moving  element,  one  causing  a  deflec- 
tion thereof  from  its  zero  position,  and  the  other,-opposing 
the  first,  limits  the  deflection  so  that  the  position  of  the 
moving  element  when  in  equilibrium  gives  a  proper  indica- 
tion of  the  magnitude  of  the  deflecting  force. 

Ammeters.  There  are  four  types  of  commercial  amme- 
ters: (i)  those  in  which  the  force-action  between  two  coils 
carrying  current  serves  as  a  measure  of  that  current,  (2) 
those  in  which  the  force-action  between  a  permanent  mag- 
net and  a  coil  carrying  current  is  utilized  as  a  measure  of 
the  current,  (3)  those  in  which  the  amount  of  attraction  of 
a  soft -iron  core  or  vane  by  a  coil  carrying  current  serves 
as  an  indication  of  the  current  strength,  and  (4)  those 
in  which  the  expansion  of  a  wire  heated  by  the  passage 


306  DYNAMO   ELECTRIC    MACHINERY. 

of  a  current  through  it   is  utilized  as  a  measure   of  the 
current. 

The  electrodynamometer  is  an  ammeter  of  the  first  type, 
and  consists  of  two  coils  connected  in  series,  one  coil  being 
fixed  and  the  other  movable.  The  planes  of  these  coils  are 
normally  at  right  angles  to  each  other,  but  when  a  current 
flows  through  them  they  tend  to  place  themselves  in  the 
same  plane.  This  tendency  of  rotation  of  the  movable  coil 
is  resisted  by  a  torsional  spring.  The  angle  through  which 
the  spring  is  turned  in  order  to  restore  the  coil  while  carry- 
ing the  current  to  its  original  position  is  measured  by  means 
of  a  pointer  and  a  dial.  If  a  be  the  angle  turned  through, 
then  the  current  strength  is 


where  k  is  a  constant  determined  by  calibration. 

Ammeters  of  the  second  type  may  have  the  coil  fixed 
and,  the  permanent  magnet  movable,  but  the  moving-coil 
instruments,  such  as  the  Weston  meter,  are  more  generally 
used.  The  Weston  instrument  consists  of  a  coil  composed 
of  a  large  number  of  turns  of  fine  insulated  copper  wire 
wound  on  a  light  rectangular  frame  of  copper  or  aluminum', 
this  coil  being  pivoted  in  jeweled  bearings  and  mounted  in 
an  annular  space  between  the  poles  of  a  permanent  magnet 
and  a  soft-iron  core  at  the  center,  as  shown  in  Fig.  199. 
Two  spiral  springs  serve  to  carry  the  current  to  and  from 
the  moving  coil,  and  also  control  the  amount  of  deflection. 
The  current  strength  is  directly  proportional  to  the  angle 
of  deflection,  and  therefore  the  scales  of  such  instruments 
are  uniform  over  the  entire  range. 

Ammeters  in  which  a  soft-iron  piece  is  attracted  by 
an  electromagnet  carrying  the  current  to  be  indicated  are 


CENTRAL-STATION    EQUIPMENT.  307 

most  generally  used  for  the  measurement  of  alternating 
currents,  but  serve  equally  well  for  direct  currents.  In  the 
most  approved  form 
of  this  type  of  instru- 
ment the  current  to 
be  measured  passes 
through  a  fixed  coil 
and  thereby  magne- 
tizes a  soft -iron  vane 
which  is  pivoted  and 
controlled  by  a  spring. 
Magnetizing  the  vane 
causes  it  to  move, 
the  amount  of  move- 

Fig.  199. 

ment   indicating  the 

current  strength   on  the  properly  calibrated  scale. 

The  principle  of  operation  of  the  hot-wire  type  of  amme- 
ter is  that  the  heat  produced  by  a  current  which  traverses 
a  wire  having  a  negligible  temperature  coefficient  of  resist- 
ance is  proportional  to  the  square  of  that  current, ^nd,  since 

the  temperature  rise  of  the  wire 
__  is   proportional   to   the   heating, 
and  the  linear  expansion  of  the 
wire  is  proportional  to  the  tem- 

I        X-JN     \  perature  rise,  it  follows  that  the 

^-^      \  linear  expansion  is  directly  pro- 

portional  to   the   square  of  the 
current   value.     The    expansion 

of  the  wire  may  be  observed  by  a  pointer,  P,  which  passes 
over  a  calibrated  scale,  the  arrangement  being  as  shown 
in  Fig.  200. 

Frequently  only  a  small  part  of  the  whole  current  to  be 


308 


DYNAMO    ELECTRIC    MACHINERY. 


measured  passes  through  the  ammeter  coil,  the  remainder 
flowing  through  a  by-path  of  low  resistance,  called  a  shunt. 
The  resistance  of  a  shunt  for  a  given  instrument  is  so  pro- 
portioned that  a  full-scale  deflection  will  be  produced  when 
a  specified  current  flows  through  the  shunt.  The  size  of 
the  shunt  is  designated  by  this  current  value.  If  it  be 
proposed  to  use  a  moving-coil  instrument,  which  gives  a 
full-scale  deflection  on  E  volts,  to  indicate  a  maximum  cur- 
rent of  /  amperes,  then  the  resistance  of  the  shunt  must 

be  —  ohm. 

Voltmeters.     Most   voltmeters  are  simply  ammeters  of 
very  high   resistance.     They  may  therefore  be  connected 

to  supply  mains  without 
causing  more  than  a  slight 
flow  of  current  through  the 
instrument.  The  resist- 
ance of  voltmeters  is  in  the 
neighborhood  of  100  ohms 
per  volt  of  maximum  scale 
deflection.  The  appearance 
of  switchboard  voltmeters 
and  ammeters  is  shown  in 
Fig.  20 1. 

Another  type  of  volt- 
meter, depending  for  its  operation  on  the  attraction  between 
two  electrically  charged  bodies,  is  the  electrostatic  volt- 
meter. Low- voltage  instruments  of  this  type  consist  of  a 
number  of  thin  plates  horizontally  suspended  between 
corresponding  quadrants,  and  fitted  with  a  pointer  which 
plays  over  a  divided  scale. 

In  order  to  vary  the  range  of  the  usual  type  of  voltmeters, 


CENTRAL-STATION    EQUIPMENT. 


309 


resistance  is  connected  in  series  with  the  instruments,  such 
resistances  being  known  as  miiltipliers.  If  R  be  the  resist- 
ance of  a  low-reading  voltmeter,  and  r  be  that  of  the  multi- 
plier, then  the  range  of  the  instrument  has  been  increased 

— — —  times.     Multipliers  should  be  wound  non-inductively 

with  wire  having  a  negligible  temperature  coefficient  of 
resistance. 

Wattmeters.  The  power  delivered  to  direct-current  re- 
ceiving circuits  may'be  determined  from  voltmeter  and 
ammeter  readings,  or  may  be  measured  directly  by  means 
of  a  wattmeter.  This  instrument  consists  of  a  fixed  coil, 
which  is  connected  in  series  with  the  load  circuit,  and  a 
movable  coil,  which  is  connected  in  series  with  a  high  re- 
sistance across  the  supply  mains,  as  shown  in  Fig.  202. 


Fig.  202. 

The  deflecting  force  is  proportional  to  the  product  of  the 
currents  flowing  in  the  two  windings,  but  the  current  flow- 
ing in  the  movable  coil  is  proportional  to  the  voltage  of  the 
circuit;  therefore  the  deflecting  force  is  proportional  to 
the  product  of  the  current  supplied  and  the  voltage,  or  to 
the  power  delivered  to  the  load.  Wattmeters  are  used 
principally  in  connection  with  alternating-current  measure- 
ments. 

To  measure  the  energy  delivered  to  a  circuit,  watt-hour 


3io 


DYNAMO   ELECTRIC    MACHINERY. 


meters  are  employed.  In  order  that  an  instrument  may 
record  the  time  element,  some  part  of  the  meter  must  move 
constantly  through  unit  distance  for  each  unit  of  energy 
delivered,  and  this  movement  must  be  permanently  recorded 
by  a  suitable  device  such  as  a  dial  train.  The  Thomson 
watt-hour  meter,'  shown  in  Fig.  203,  consists  of  a  spherical 

armature  rotating  within 
two  circular  field  coils,  one 
on  either  side  of  the  arm- 
ature. This  instrument  is 
connected  to  the  circuit  in 
the  same  manner  as  the 
indicating  watt-meter.  The 
armature  is  carried  by  a 
vertical  spindle,  the  lower 
end  of  which  rests  in  a  jew- 
eled bearing,  and  the  upper 
end  is  provided  with  a  worm 
which  meshes  with  a  chain 
of  wheels  constituting  the 
counting  mechanism.  The 
torque  produced  is  propor- 
Flg*  2°3'  tional  to  the  product  of  the 

field  flux  and  the  armature  current.  As  there  is  no  iron  in 
the  magnetic  circuit  and  since  the  speed  of  rotation  is  low, 
little  or  no  counter  E.M.F.  will  be  induced  in  the  armature 
coil;  thus  the  armature  current  is  independent  of  the  speed, 
and  is  directly  proportional  to  the  line  voltage.  For  the 
same  reason  the  field  flux  is  proportional  to  the  main  current. 
Therefore  the  torque  is  proportional  to  the  power  consumed 
by  the  load.  In  order  that  the  meter  shall  record  correctly 
it  is  only  necessary  to  provide  some  means  for  making  the 


CENTRAL-STATION    EQUIPMENT. 


speed  of  rotation  proportional  to  the  torque.  This  is  accom- 
plished by  applying  a  magnetic  drag,  in  the  form  of  an 
aluminum  disk  fastened  to  the  armature  spindle  and  passing 
between  the  poles  of  permanent  magnets.  The  electromo- 
tive forces  induced  in  the  disk  are  proportional  to  the 
number  of  lines  of  force  cut  in  a  given  time,  and,  since  the 
resistance  of  the  disk  is  constant,  the  strength  of  the  eddy 
currents  will  be  proportional  to  the  rate  of  cutting  lines  of 
force,  and  consequently  will  vary  with  the  speed  of  rotation. 
The  drag  or  counter  torque,  being  proportional  to  the  prod- 
uct of  the  constant  flux  and  the  eddy  currents,  will  vary 
directly  with  the  speed.  To  overcome  mechanical  friction 
an  auxiliary  field  coil  or  starting  coil  is  provided  which 
consists  of  a  few  turns 
of  fine  wire  connected 
in  series  with  the  arma- 
ture. 

Instruments  for  re- 
cording successive  in- 
stantaneous values  of 
current,  voltage  or 
power  are  called  record- 
ing instruments.  They 
operate  on  the  same 
principles  as  indicating 
instruments,  and  are 
made  recording  by  fit- 
ting the  pointer  with  an 
ink  pen  which  presses  against  a  paper  chart  wound 
on  a  drum,  the  latter  rotating  slowly  at  constant  speed 
by  clockwork.  A  curve-drawing  wattmeter  is  shown  in 
Fig.  204. 


Fig.  204. 


312  DYNAMO   ELECTRIC   MACHINERY. 

120.  Switchboards. — The  object  of  a  central-station 
switchboard  is  to  group  the  necessary  devices  for  controlling, 
distributing  and  measuring  the  current  received  or  delivered, 
particular  attention  being  directed  toward  locating  these 
devices  for  convenient  operation.  Safety  apparatus  for  pro- 
tecting generators  or  the  lines  against  abnormal  conditions 
are  sometimes  placed  upon  the  switchboard. 

Switchboards  are  designed  with  a  view  to  a  symmetrical 
arrangement  of  the  apparatus  and  instruments,  and  it  is 
usual  to  place  all  similar  devices  in  the  same  horizontal  row. 
As  a  rule  circuit  breakers  are  located  at  the  top  of  the 
board  and  recording  instruments  at  the  bottom.  Measuring 
instruments  are  placed  at  such  height  as  to  be  conveniently 
read  by  the  switchboard  attendant.  Rheostats  may  be 
placed  above  or  below  the  switchboard  at  the  rear,  but  the 
handles  controlling  them,  through  the  agency  of  chains  and 
sprocket  wheels,  must  be  located  so  that  the  attendant  can 
manipulate  them  and  note  the  instrument  indications  simul- 
taneously. The  bus-bars  and  the  connections  from  them  to 
the  switches,  circuit  breakers,  rheostats  and  to  the  instru- 
ments are  mounted  on  the  rear  of  the  switchboard. 

Switchboards  are  constructed  so  that  each  panel  controls 
the  apparatus  for  a  single  generator-  or  controls  a  definite 
number  of  feeders.  Fig.  205  illustrates  a  switchboard  for 
two  compound-wound  generators,  and  consisting  of  two 
generator  panels  and  one  feeder  panel  for  four  feeders. 
Each  generator  may  be  connected  to  the  bus-bars  by  means 
of  a  main  switch  and  a  circuit  breaker.  The  current  sup- 
plied to  the  load  by  each  generator  is  measured  by  a 
separate  ammeter  and  shunt,  but  the  voltage  of  both  ma- 
chines is  measured  alternately  by  a  single  voltmeter  which 
may  be  connected  to  either  machine  by  a  voltmeter  switch 


CENTRAL-STATION    EQUIPMENT. 


313 


Fig.  205. 


314  DYNAMO    ELECTRIC   MACHINERY, 

on  the  feeder  panel.  The  rheostats  on  the  generator  panels 
regulate  the  excitation  of  the  generator  shunt  fields.  In 
order  that  these  compound-wound  machines  may  be  oper- 
ated in  parallel,  equalizer  switches  and  an  equalizer  bus-bar 
are  necessary.  A  lamp  on  each  panel  provides  illumination 
for  the  scales  of  the  measuring  instruments;  the  lamps  on 
the  generator  panels  also  serve  as  pilot  lamps.  The  outer 
lamps  on  the  feeder  panel  constitute  a  ground  detector,  and 
are  used  for  indicating  grounds.  Should  a  partial  ground 
occur  on  one  line  the  corresponding  lamp  would  burn  dimly 
and  the  other  brightly,  thus  indicating  by  their  relative 
brightness  the  extent  of  the  fault. 

121.  Works  Cost.  —  The  costs  of  the  various  items 
which  are  involved  in  the  manufacture  of  dynamo-electric 
machines  are  so  dependent  upon  the  types  of  the  specific 
designs  that  a  comprehensive  discussion  of  them  cannot  be 
given  in  this  book.  Commercial  designers,  however,  are 
responsible  for  the  manufacture  of  machines  which  are  as 
inexpensive  as  is  consistent  with  satisfactory  operation. 
Although,  with  a  machine  of  given  speed  and  output,  the 
detail  costs  for  labor,  iron,  copper  and  insulating  material 
may  vary  widely  in  different  cases,  the  total  works  costs 
are  not  very  different.  If  the  diameter  and  over-all  length 
of  the  armature  be  D  and  /  inches  respectively,  then  the 
works  cost  in  dollars  may  be  expressed  as 

Cost  =  KDl, 

where  K  is  a  function  of  the  speed  and  output  of  the 
machine.  This  function  is  fairly  constant  for  pressures 
between  looand  500  volts,  for  the  extra  costs  of  insulation 
in  the  construction  of  the  machines  of  higher  voltage  are 
compensated  for  by  the  reduced  costs  of  commutator  copper. 


CENTRAL-STATION    EQUIPMENT. 


315 


The  values  of  K  may  be  obtained  from  the  curves  shown 
in  Fig.  206,  which  correspond  to  peripheral  velocities,  v,  of 
the  armature  of  200,  150,  and  100  or  less  feet  per  second. 
The  increased  values  for  velocities  above  100  are  due  to 
increased  costs  of  labor  and  of  suitable  material  for  with- 
standing the  higher  centrifugal  forces. 


400  600 

OUTPUT  IN  K.W. 

Fig.  206. 


soo 


1000 


122.  Selling  Prices.  —  The  selling  price  of  a  machine 
must  exceed  the  works  cost  by  an  amount  sufficient  to 
include  the  profit  and  such  expenses  as  litigation,  advertis- 
ing, and  sales  commissions  or  expenses.  It  is  customary  for 
manufacturing  concerns  to  print  in  their  commercial  publi- 
cations list-prices  of  machines  which  considerably  exceed 
the  total  costs  of  them  when  delivered.  Substantial  dis- 
counts are  allowed  to  purchasers,  amounting  to  as  much 
as  50  %  in  some  cases,  and  depending  upon  the  extent  of 
purchase,  date  of  payment  and  many  other  complex  condi- 
tions. The  actual  selling  price  per  K.W.  is  dependent  upon 
the  speed,  output  and  type  of  machine.  To  give  a  general 
idea  as  to  the  selling  prices  of  compound-wound  generators 
the  curves  of  Figs.  207  and  208  are  given.  The  prices  are 
not  far  from  those  which  would  be  paid  by  unfavored  pur- 


316 


DYNAMO   ELECTRIC    MACHINERY. 


100  150 

CAPACITY  IN    K.W. 

Fig.  207. 


PRICE  PER  K.W.  IN  DOLLARS 

5  §  £ 

V 

250-VOLT 
ENGINE-TYPE 
COMPOUND  GENERATORS 
AT  100  R.P.M. 

\ 

X 

Sv- 

^ 

"•^^ 

^^**, 

~^** 

*—     — 

—       — 

—  —  . 

)                        200                     400                      600                     800                    1(X 
CAPACITY  IN    K.W. 

Fig.  208. 


CENTRAL-STATION    EQUIPMENT. 


317 


chasers  in  1910.  In  Fig.  209  are  given  the  selling  prices 
of  23o-volt  shunt  or  series  motors  for  normal  speeds  of 
1000  revolutions  per  minute,  and  of  125-volt  balancers. 


i 


X 


30 
CAPACITY  IN    K.W. 

Fig.  209. 


1000  2000  3000  4000 

AMPERE-HOUR  CAPACITY  AT  8-HOUR  RATE 


Fig.  210. 


The  selling  prices  of  lead-lead-sulphuric  acid  storage  cells 
per  ampere-hour  of  capacity  at  an  8-hour  rate  are  em- 
bodied in  the  curve  of  Fig.  210  as  functions  of  the 
capacities. 


318  DYNAMO   ELECTRIC   MACHINERY. 

123.  Plant  Costs.  —  A  fair  average  cost  per  kilowatt  for 
a  steam-driven  power  plant  is  $100,  although  with  steam- 
turbine  plants  it  may  be  placed  at  $80.  For  hydraulic 
plants  the  cost  is  greater  than  these,  and  has  been  estimated 
at  $200  per  kilowatt.  These  prices  are  applicable  to  central 
stations  of  reasonably  large  capacity.  In  such  stations 
alternating-current  generators  are  usually  employed.  Di- 
rect-current generators  are  more  generally  used  in  isolated 
plants,  such  as  in  office  buildings  or  in  manufactories,  where 
steam  is  generated  for  heating  or  power  purposes  and  its 
use  for  driving  electrical  generators  is  subsidiary  and  inci- 
dental. In  such  cases  the  cost  of  delivery  and  of  erection 
of  generators  may  be  estimated  as  60  cents  per  K.W. 
The  various  installation  costs  may  be  estimated  from  the 
data  given  by  Stott  in  the  following  table: 

POWER   PLANT    COST    PER  K.W. 

Minimum  Maximum 

1 .  Real  Estate $3  .  oo  $7 .  oo 

2.  Excavation .75  i .  25 

3.  Foundations,  Reciprocating  Engines 2.00  3-°o 

4.  Foundations,  Turbines .50  .75 

5.  Iron  and  Steel  Structure   8.00  10.00 

6.  Building  (Roof  and  Main  Floor) 8.00  10.00 

7.  Floors,  Galleries  and  Platforms i .  50  2  . 50 

8.  Tunnels,  Intake  and  Discharge i  .40  2  .80 

9.  Ash  Storage  Pocket,  etc 70  i .  50 

10.  Coal-hoisting  Tower i .  20  2  .  oo 

u.   Cranes .40  .60 

1 2.  Coal  and  Ash  Conveyors 2 .  oo  2.75 

13.  Ash  Cars,  Locomotives  and  Tracks .15  .30 

14.  Coal  and  Ash  Chutes,  etc .40  i  .00 

15.  Water  Meters,  Storage  Tanks  and  Mains  .           .50  i  .00 

1 6.  Stacks 1.25  2  .  oo 

17.  Boilers 9.50  11.50 

18.  Boiler  Setting 1.25  1.75 


CENTRAL-STATION   EQUIPMENT.  319 

Minimum  Maximum 

19.  Stokers $1.30  $2.20 

20.  Economizers i . 30  2.25 

21.  Flues,  Dampers  and  Regulators .60  .90 

22.  Forced  Draught  Blowers,  Air  Ducts,  etc.  .  1.25  *-6$ 

23.  Boiler  Feed  and  Other  Pumps .40  .75 

24.  Feed-water  Heaters,  etc .20  .35 

25.  Steam  and  Water  Piping,  Traps,  Separators, 

High  and  Low  Pressure 3 .00  5 .00 

26.  Pipe  Covering 60  i .  oo 

27.  Valves .60  i  .  oo 

28.  Main  Engines,  Reciprocating 22  .00  30.00 

29.  Exciter  Engines,  Reciprocating .40  .70 

30.  Condensers,  Barometric  or  Jet i  .00  2.50 

31 .  Condensers,  Surface 6 .  oo  7  . 50 

32.  Electric  Generators 16.00  22.00 

33.  Exciters 60  .80 

34.  Steam  Turbine  Units  Complete 22  .  oo  32  .00 

35.  Rotaries,  Transformers,  Blowers,  etc .60  i  .00 

36.  Switchboards    Complete 3.  oo  3. 90 

37.  Wiring  for  Lights,  Motors,  etc .20  .30 

38.  Oiling  System  Complete .15  .35 

39.  Compressed  Air  System  and    Other   Small 

Auxiliaries ' .20  .30 

40.  Painting,  Labor,  etc 1.25  1.75 

41.  Extras 2  .00  -  2  .00 

42.  Engineering  Expenses  and   Inspection 4.00  6.00 

Costs  of  Excavation  near  New  York  City 

Earth $2.44  per  cu.  yd. 

Rock 6 .  oo  per  cu.  yd. 

Brickwork 1 1 . 30  per  cu.  yd. 

124.    Operating  Expenses There  are  many  items  which 

make  up  the  expenses  for  operating  a  plant  and  for  making 
repairs  which  are  essential  for  maintaining  the  machinery 
in  good  running  condition.  The  relative  costs  of  these 
items  have  been  given  by  Stott  as  listed  in  the  following 
table : 


320 


DYNAMO   ELECTRIC   MACHINERY. 


DISTRIBUTION    OF   MAINTENANCE   AND    OPERATING 
CHARGES. 


o 

c     % 

9 

Z  2   Z 

ft     2 

<» 

ll 

s  1 

O      M     i- 

J 

lei 

a.  z 

*z: 

w  J 

w  < 

H 

§a< 

Ex] 

W     i   "^ 

O        K 

X 

K     Z 

a 

MAINTENANCE 

i.  Engine  Room  Mechanical    . 
2.  Boiler  Room  or  Producer  Room 

2-57 
4.61 

0.51 
4.30 

1.54 

3-52 

2-57 

1.54 
i-95 

3.  Coal-  and  Ash  -handling  Appa- 

oc8 

O.C4 

O  4-4 

0.  2O 

o  29 

4.  Electrical  Apparatus   .... 

1.  12 

1.  12 

1.  12 

1.  72 

OPERATION 

5.  Coal-  and  Ash-handling  Labor  . 
6.  Removal  of  Ashes      .... 

2.26 
1.  06 

2.  II 

o-94 

1.74 
0.8o 

I-I3 
0-53 

0.50 

7    Dock  Rental           .                .     . 

o  74 

o  74 

O  74 

O  74 

0.74 

8.    Boiler-room  Labor     .... 

7.15 

6.68 

S-46 

1.79 

303 

9.  Boiler-room  Oil,  Waste,  etc.     . 

0.17 

0.17 

0.17 

0.17 

0.17 

10    Coal    .          ... 

61  10 

r-   -jo 

4687 

26.71 

2^77 

1  1    Water       .     . 

7.14 

O  71 

c  46 

7    C7 

2  14 

12.  Engine-room  Mechanical  Labor 

6.71 

4.03 

6.71 

4.03 

13.  Lubrication    

1.77 

0.35 

I.OI 

1.77 

1.  06 

14    Waste,  etc.     ... 

O.  "iO 

o  10 

O  7O 

15    Electrical  Labor 

2.Z2 

2.^2 

2.C2 

2  52 

2  K2 

Relative  Cost  of  Maintenance  and 

Operation 

IOO    OO 

70  64 

7C  72 

5067 

46  12 

Relative  Investment  in  per  cent 

100.00 

82.50 

77.00 

100.00 

91.20 

125.  Cost  of  Electrical  Energy The  cost  attendant 

upon  the  delivery  of  electrical  energy  at  the  bus-bars  of  a 
central  station  is  made  up  of  two  factors. 

The  first  is  constant  in  magnitude,  is  independent  of  the 
amount  of  delivered  energy,  and  appears  in  the  station 
records  as  *  fixed  charge.  It  includes  such  items  as  interest 
on  the  investment,  insurance,  taxes,  depreciation,  and  obso- 
lescence. The  last  item  is  one  which  is  due  to  the  frequent 


CENTRAL-STATION    EQUIPMENT. 


321 


improvements  in  central  station  apparatus,  which  make  it 
desirable  at  times  to  cast  aside  operative  machines  before 
they  are  worn  out,  in  order  to  take  advantage  of  the  greater 
efficiency  afforded  by  newer  types.  Dr.  C.  T.  Hutchinson 
gives  the  following  approximate  values  for  the  items  of  fixed 
charges  expressed  as  percentages  of  the  cost  of  the  plant: 

FIXED    CHARGES    OF   GENERATING    PLANTS. 


STEAM 

WATER 

Interest 

6.0% 

6.0% 

Insurance 

o.<? 

Taxes                   

o.q 

QC 

Depreciation       
Obsolescence      

5-o 
5.0 

1.0 

'•5 

i7-o% 

9-o% 

The  second  factor  is  the  maintenance  and  operating 
expense,  which  depends  upon  the  amount  of  electrical  energy 
delivered  to  the  bus-bars  and  varies  with  it.  Although  this 
expense  per  kilowatt-hour  varies  somewhat  with  the  ratio 
of  the  load  to  the  maximum  capacity  of  the  plant,  it  may 
safely  be  estimated  at  0.5  cent. 

The  average  cost  throughout  a  day  or  year  of  a  unit  of 
delivered  electrical  energy,  therefore,  depends  upon  the  ratio 
of  the  average  power  to  the  maximum  power.  This  ratio 
is  termed  the  load  factor  of  the  plant.  Inasmuch  as  all 
generators  have  an  overload  capacity,  due  to  their  ability 
to  store  heat  for  a  limited  time  without  an  excessive  result- 
ant rise  of  temperature,  the  maximum  power  is  really  taken 
as  the  average  power  for  say  an  hour  during  the  period  of 
maximum  load.  It  is  common  for  load  factors  to  be  as  low 
as  10%,  although  that  of  the  Interborough  Rapid  Transit 
Company  varies  between  50%  and  55  %.  The  influence 
of  the  load  factor  upon  the  cost  of  a  kilowatt-hour  of 


322 


DYNAMO   ELECTRIC   MACHINERY. 


electrical  energy  is  shown  in  the  following  table,  the  cal- 
culations being  based  upon  a  IOOO-K.W.  steam-turbine 
plant  costing  $80  per  kilowatt  and  operating  continuously 
throughout  the  year,  viz.  for  8760  hours. 

EFFECT    OF   LOAD   FACTOR    ON    COST   OF   POWER. 


COSTS 

LOAD    FACTORS 

O.2 

0.4 

0.6 

0.8 

I.O 

Fixed  Charges    
Operating  Expenses     
Total  per  Kilowatt-hour  in  Cents  . 

0-775 

0.500 

0.388 

0.500 

0.258 

0.500 

0.194 

0.500 

0.155 
0.500 

1-275 

0.888 

0.758 

0.694 

0.655 

PROBLEMS. 

1.  Design  a  good  5oo-ampere,  no-volt,  double-pole,  single- 
throw,  back-connected  switch,  but  use  no  more  material  than 
necessary.     How  many  pounds  of  copper  are  required  if  the 
terminal  studs  be  5  inches  long  ? 

2.  A  moving-coil  instrument  gives  a  full-scale  deflection  on 
0.068    volt.     What    is    the    resistance  of   a   2oo-ampere  shunt 
therefor  ? 

3.  The  resistance  of  a   i5o-volt  voltmeter  is  14,000  ohms. 
What  must  be  the  resistance  of  a  multiplier  for  this  instrument 
so  that  it  can  measure  E.M.F^  up  to  750  volts  ? 

4.  A  watt-hour  meter  without  a  starting  coil  reads  correctly 
when  run  on  a  i-K.W.  load.     On  light  load  a  power  consump- 
tion of  100  watts  will  just  start  the  armature  rotating.     What 
will  be  the  meter  indication  after  running  2\  hours  on  a  constant 
load  of    600   watts,  assuming    running    friction  as  one-half  of 
starting  friction  ? 

5.  Lay  out  the  connections  of  a  suitable  switchboard  for  an 
isolated  plant  having  a  single  compound-wound  generator,  which 
is  intended  to  supply  three  feeder  circuits. 


INDEX. 


Absolute  electrical  units,  3. 

Action  of  a  generator,  principle  of,  45. 

motor,  principle  of,  210. 
Acyclic  dynamos,  184. 
Ageing  of  iron,  41. 
Air  gap,   magnetic  distribution  in, 

112,  119. 

Allis-Chalmers  Co.  generators,  177. 
Alternator,  the,  46. 
Ammeters,  305. 
Ammeter  shunts,  308. 
Ampere,  definition  of,  3. 

-hour,  definition  of,  3. 

-turn,  definition  of,  32. 

-turns,  exciting,  calculation  of,  96. 
for  compensating  armature  re- 
action, 1 1 6. 

Anderson  Mfg.  Co.  switch,  300. 
Angle  of  brush  lead  or  lag,  113,  216. 
Applications  of  shunt  motors,  236. 
Arc-light  generators,  85. 
Armature  bearings,  85. 

coils*  73. 

copper  loss,  148. 

core  construction,  67. 

cores,  losses  in,  146. 

equalizing  connections,  65. 

of  dynamo,  52. 
^  reaction,  no,  216. 

compensation  for,  115. 

shafts,  83. 

slots,  72. 

windings,  55. 
Automobile  motors,  258. 
Auxiliary  field  poles,  1 20. 

Balancers,  274. 

Ball-bearings  for  armatures,  86. 
Batteries,  storage,  287. 
Bearing  friction,  151. 
Bearings,  armature,  85. 
Bipolar  field  magnets,  54. 


Boosters,  279. 

Braking,  dynamic,  262. 

Brushes,  81. 

Brush  arc-light  generator,  193. 

holders,  82. 

lead  or  lag,  113,  216. 

pressure,  80. 

transition  resistance,  79. 
Burke  Electric  Co.  three- wire  gen- 
erator, 184. 

Calculation     of     exciting     ampere- 
turns,  96. 

of  reactance  voltage,  127. 
Capacity  of  a  dynamo,  140. 
Cast  iron,  magnetic  properties  of,  34. 
Characteristic  curves  of  generators, 
162,  172,  187. 

of  motors,  233,  241,  247. 
Circuit  breakers,  303. 
Circular  mil,  definition  of,  6. 
Coefficient,  economic,  158. 

of  conversion,  158.^ 

of  dispersion,  95. 

of  mutual  induction,  25. 

output,  145. 

self-induction,  24. 
Coercivity,  39. 
Commercial  efficiency,  159. 
Commutating  plane,  113. 

poles,  120,  219. 

Commutation,  conditions  for  spark- 
less,  134. 

frequency  of,  126. 

process  of,  121. 

self-inductance,  122,  129. 

time  of,  125. 
Commutator  construction,  76. 

function  of,  46. 

losses,  79,  152. 

Compensation     for     armature     re- 
action, 115. 


323 


324 


INDEX. 


Compound  excitation,  92. 
Compounding,  171. 
Compound-wound    generators,    see 

Generators, 
motors,  263. 

Conductivity,  definition  of,  6. 
Conductors,  resistance  of,  5. 
Constant-current  booster,  283. 

distribution,  160. 
-potential  distribution,  160. 
Control  of  motor  speed,  218. 

of  railway  motors,  251. 
Copper  loss  in  armature  coils,  148. 
Core  construction,  67. 
Cost  of  dynamos,  314. 
of  electrical  energy,  320. 
of  operating  machine  tools,  238. 
of  plants,  318. 
of  storage  batteries,  317. 
Coulomb,  definition  of,  3. 
Counter  E.M.F.  of  motors,  214. 
Crane  motors,  261. 
Crocker- Wheeler  mill  motor,  260. 

motor  for  lathes,  237. 
Cross-magnetizing   effect   of   arma- 
ture current,  in. 
Current,  absolute  unit  of,  3. 
density  in  field  coils,  107. 
Cutler-Hammer     Mfg.      Co.      con- 
troller, 263. 
rheostat,  169. 
starting  and  field  rheostat,  228. 

Decay  of  current  in  inductive  cir- 
cuit, 27. 

Degree  of  re-entrancy,  64. 

Demagnetizing   effect    of   armature 
current,  113. 

Density  of  flux,  14. 

Design  of  starting  rheostats,  230. 

Dettmar's  three-wire  generator,  182. 

Devices  for  reducing  armature  re- 
action, 118. 

Diamagnetic  substances,  21. 

Dielectric  strength,  n. 
test  of,  13. 

Difference  of  potential,  3. 

Differential  boosters,  282. 
motor,  263. 

Direction  of  induced  E.M.F. ,  23. 
of  rotation  of  motors,  211. 

Dispersion  coefficient,  95. 


Distribution,  constant-current,  160. 
-potential,  160. 

three- wire,  181. 
Divided  circuits,  8. 
Dobrowolsky's     three-wire     gener- 
ator, 182. 

Ducts  for  ventilation,  70. 
Dynamic  braking,  262. 
Dynamo  capacity,  140. 

homopolar,  184. 
Dynamos,  45. 

E.M.F.  equation  of,  66. 

heating  of,  142. 
Dynamotors,  266. 
Dyne,  definition  of,  i. 

Economic  coefficient,  158. 
Eddy  currents,  42. 

losses  due  to,  147. 
Electric     Mfg.     Co.     automobile 

motor,  259. 
Effect    of    load    factor   on    cost    of 

power,  322. 
Efficiency  of  dynamos,  155. 

of  motors,  234. 
Electrical  distribution,  160; 
efficiency,  159. 
energy,  cost  of,  320. 
units,  3. 
Electro-Dynamic  Co.  dynamo  field 

structure,  121. 
motor,  221. 

Electrodynamometers,  306. 
Electro-magnetic  induction,  21. 
Electromotive      force      fluctuation, 

52. 

generated,  48. 
induced  in  conductor,  23. 
of  dynamo,  66. 
Element  of  winding,  56. 
Elevator,  motor-driven,  238. 
End-cell  control,  282. 
Energy,  definition  of,  i. 
Entz  regulator,  285. 
Equalizers,  269,  298. 
Equalizing  connections,  65. 
Erg,  definition  of,  i. 
Excelsior  arc-light  generator,  198. 
Excitation  loss,  151. 

of  field  magnets,  92. 
Exciting    ampere-turns,   calculation 
of,  96. 


INDEX 


325 


Expenses,  operating,  319. 
External  characteristic,  187. 

Feeders,  166. 
Field  coils,  53,  104. 

current  density  in,  107. 

cores,  88. 

excitation,  92. 

magnet  frames,  88,  180. 

magnets,  53. 

rheostats,  166. 
Flat  compounding,  172. 
Fleming's  Rule,  23,  211. 
Fluctuation  of  E.M.F.,  52. 
Flux  density,  20. 
Foot-pound,  definition  of,  i. 
Force,  definition  of,  i. 
Foucault  currents,  42. 
Frequency,  52. 

of  commutation,  127. 
Function  of  commutator,  46. 
Fuses,  302. 

Gauss,  definition  of,  21. 
General   Electric  Co.   constant-cur- 
rent generator,  193. 
constant-potential       generator, 

174,  179,  184. 
field  rheostat,  167. 
railway  motor,  247. 
shop-tool  controller,  229. 
Generated  electromotive  force,  48. 
Generator  losses,  146. 
output  of,  145. 
principle  of  action  of,  45. 
Generators,  compound- wound,  char- 
acteristic curves  of,  172. 
efficiency  of,  155. 
field  excitation  of,  92. 
for  lighting  and  railways,  173. 
homopolar,  184. 
parallel  operation  of,  294. 
rating  of,  145. 
series-wound,  189. 

characteristic  curves  of,  187. 
shunt-wound,  characteristic  curves 

of,  162. 

regulation  of,  164. 
split-pole,  1 1 8. 
three- wire,  180. 
Gilbert,  definition  of,  32. 
Ground  detectors,  314. 


Growth  of  current  in  inductive  cir- 
cuit, 26. 

Hamilton  motor-operated  drill  press, 

236. 

Hand  regulation,  165. 
Heat  developed  by  a  current,  n. 
Heating  of  dynamos,  142. 
Henry,  definition  of,  25. 
Homopolar  generators,  184. 
Horse-power,  definition  of,  2. 
Hot-wire  instruments,  307. 
Hubbard  booster  system,  284. 
Hysteresis    loss  in  armature  cores, 

147. 

magnetic,  38. 
Hysteretic  constants,  4^. 

Induction,  20. 

electro-magnetic,  21. 

mutual,  25. 

self,  24. 

Inductors,  armature,  56. 
Industrial     applications     of     shunt 

motors,  236. 
Instruments,  305. 
Insulating  materials,  n. 
Insulation  resistance,  n. 
Intensity  of  magnetic  field,  19. 
Interpoles,  120,  219. 
Iron  loss  in  armature  cores,  147. 

Joule,  definition  of,  i. 

Kinetic  energy,  definition  of,  2. 
Kirchhoff's  Laws,  9. 

Laminations  of  armature  core,  68. 

Lap  armature  windings,  57. 

Leakage,  magnetic,  94. 

Lighting  generators,  173. 

Lines  of  force,  18. 

Load  factor  of  plants,  321. 

Loading-back     method     of    testing 

motors,  235. 
Losses  in  armature  coils,  148. 

cores,  146. 

in  commutator  and  brushes,  153. 
in  field  pole  faces,  149. 

winding,  151. 
Lubrication,  85. 
Lundell  generator,  118,  179. 


326 


INDEX. 


Machine-tool     operation,    cost    of, 

238. 
Magnetic   distribution    in   air   gap, 

112,  119. 
Magnetic  field,  definition  of,  18. 

intensity  of,  19. 
hysteresis,  38. 
leakage,  94. 
permeability,  20. 
potential,  20. 

properties  of  iron  and  steel,  34. 
Magnetization  curves,  33. 
Magneto,  92,  159. 
Magnetomotive  force,  32. 
Magnet  pole,  strength  of,  18. 
Matthiessen  standard  resistivity,  7. 
Maxwell,  definition  of,  21. 
Measuring  instruments,  305. 
Mechanical  units,  i. 
Meters,  recording  watt-hour,  310. 
Mill  motors,  260. 

Motor,  armature  reactions  of,  216. 
counter  E.M.F.  of,  214. 
direction  of  rotation  of,  211. 
-generators,  273. 
power  of,  216. 
principle  of  action  of,  210. 
torque  exerted  by,  213. 
Motors,  compound- wound,  263. 
field  excitation  of,  92. 
parallel  operation  of,  299. 
series,  239. 

characteristic  curves  of,  241. 
for  automobiles,  258. 
for  rolling  mills,  260. 
on  railways,  243. 

shunt,  characteristic  curves  of,  233. 
industrial  applications  of,  236. 
interpole,  221. 
speed,  217. 

control  of,  222,  277. 
regulation  of,  232. 
starting  of,  225. 
Multiple-circuit  armature  windings, 

57- 

-unit  railway  motor  control,  255. 
Multiplex  armature  windings,  62. 
Multipliers,  309. 
Multipolar  field  magnets,  55. 
Multivoltage    distribution    system, 

222. 
Mutual  induction,  25. 


Negative  booster,  280. 

Neutral  of  three- wire  system,  181. 

plane,  115. 
Non-reversible  booster,  283. 

Oersted,  definition  of,  37. 
Ohm,  definition  of,  4. 
Ohm's  Law,  4. 
Operating  expenses,  319. 
Otis  traction  elevator,  238. 
Output  coefficients,  145. 
Over-compounding,  172. 

Parallel  connection  of  circuits,  8. 

operation  of  motors,  299. 
Paralleling  of  generators,  294. 
Paramagnetic  substances,  21. 
Permeability,  20. 
Permeance,  37. 
Pitch,  pole,  90. 

winding,  59. 

Plane  of  commutation,  113. 
Polar  span,  91. 
Pole- face  losses,  149. 

pitch,  90. 

shoes,  88. 
Potential  energy,  definition  of,  2. 

magnetic,  20. 
Poundal,  definition  of,  i. 
Pound  as  a  unit  of  force,  i. 
Power,  definition  of,  2. 

lines,  189. 

of  electric  current,  10. 

of  motors,  216. 

plant  costs,  318. 

transmission,  Thury  system,  296. 
Practical  electrical  units,  3. 
Pressure,  definition  of,  4. 
Principle  of  generator  action,  45. 

of  motor  action,  210. 
Problems,  16,  43,  86,  108,  138,  207, 

265,  292,  322. 
Protective  devices  in  circuits,  302. 

Quantity  of  electricity,  3,  29. 

Railway  generators,  173. 
motors,  243. 

characteristic  curves  of,  247. 
control  of,  251. 


INDEX. 


327 


Rating  of  machines,  145. 
Reactance  voltage,  123. 
calculation  of,  127. 
Reaction  of  armature  currents,  no. 
compensation  for,  115. 
devices  for  reducing,  118. 
Recording  instruments,  311. 
Re-entrant  armature  windings,  63. 
Regulation  of  series  generators,  190. 

speed  of  shunt  motors,  232. 

voltage  of  generators,  164. 
Reluctance,  36. 
Reluctivity,  definition  of,  37. 
Resistance,  4. 

specific,  6. 

temperature  coefficient  of,  7. 
Resistivity,  definition  of,  5. 
Retentivity,  39. 
Reversible  booster,  283. 
Rheostatic  railway  controller,  251. 
Rheostats,  field,  166. 

starting,  225. 

design  of,  230. 
Rolling  mills,  motors  for,  260. 

Saturation,  33. 
Self-induction,  25. 

-regulation,  172. 
Selling  prices  of  machines,  315. 
Series-booster,  279. 

connection  of  circuits,  7. 

excitation,  92. 

-parallel  railway  controller,  252. 

-wound  generators,  see  Generators. 
Shafts  of  armatures,  83. 
Shop-tool  controller,  229. 
Short-chord  armature  windings,  62. 
Shunt-booster,  281. 

excitation,  92. 

motors,  see  Motors. 
Shunts  for  ammeters,  308. 
Shunt-wound  generators,   see   Gen- 
erators. 

Simplex  armature  windings,  56. 
Slots  in  armature  cores,  72. 
Space  factor  of  armature  slots,  74. 

of  field  coils,  107. 
Sparkless   commutation,   conditions 

for,  134. 

Specific  resistance,  6. 
Speed  control  of  shunt  motors,  218. 

regulation  of  shunt  motors,  232. 


Split-pole  type  of  generator,  118. 
Starting  rheostats,  225. 

design  of,  230. 
Steam-turbine      driven      generator, 

179. 

Steel,  magnetic  properties  of,  35. 
Storage  batteries,  287. 

cost  of,  317. 

Stow  Mfg.  Co.  motor,  221. 
Strength,  dielectric,  n. 

of  magnet  pole,  18. 
Suspension    of    railway    motors    on 

trucks,  250. 
Switchboards,  312. 
Switches,  300. 

Table  of  armature  shaft  fits,  174. 
cost  of  machine-tool  operation, 

238. 

current  paths  in  armature  wind- 
ings, 67. 

dielectric  strengths,  14. 
fixed  charges  of  plants,  321. 
flux  densities  in  dynamos,  97. 
generator  speeds,  141,  174. 
hysteretic  constants,  41. 
load  factor  versus  cost  of  power, 

322. 
magnetic  properties  of  iron  and 

steel,  35. 
number  of  dynamo  field  poles, 

142. 
operating      and      maintenance 

charges,  320. 
power  plant  cost,  318. 
resistivities,  6. 
switch  data,  301. 
test  voltages,  16. 

Temperature   coefficient    of    resist- 
ance, 7. 

elevation  of  dynamos,  143,  153. 
Thomson-Houston     arc-light     gen- 
erator, 200. 
watt-hour  meter,  310. 
Three- wire  generators,  180. 
Thury  system  of    power    transmis- 
sion, 296. 

Time  of  commutation,  125. 
Toroids,  32. 

Torque  exerted  by  motors,  213. 
Track-return  booster,  280. 


328 


INDEX. 


Unipolar  dynamos,  184. 

Variable-speed  control  of  motors,  218. 
Ventilating  ducts  in  core,  70. 
Voltage    regulation    of    shunt    gen- 
erators, 164. 
Volt,  definition  of,  3. 
Voltmeter  multipliers,  309. 
Voltmeters,  308. 

Ward  Leonard  motor  control  system, 

223. 
self-starter,  229. 

Watt,  definition  of,  2. 

Wattmeters,  300. 

Wave  armature  windings,  57. 

Western  Electric  Co.  arc-light  gen- 
erator, 204. 


Western     Electric     Co.     generator. 

176. 
Westinghouse  circuit  breaker,  304. 

field  rheostat,  168,  169. 

generator,  178. 

railway  controller,  254. 

motor,  243. 

Weston  instruments,  306. 
Windage,  151. 
Winding  pitch,  59. 
Windings,  armature,  55. 
Work,  definition  of,  i. 
Works  cost,  314.  t 

Wrought  iron,   magnetic  properties 
of,  34- 

Yoke,  54- 


LIST  OF  WORKS 


ON 


ELECTRICAL    SCIENCE 

PUBLISHED   AND   FOR    SALE   B\ 

D.    VAN   NOSTRAND   COMPANY, 

23  Murray  and  27  Warren  Streets,  New  York* 


ABBOTT,  A.  V.  The  Electrical  Transmission  of  Energy.  A  Manual  for 
the  Design  of  Electrical  Circuits.  Fifth  Edition,  enlarged  and  rewritten. 
With  many  Diagrams,  Engravings  and  Folding  Plates.  8vo.,  cloth, 
675  pp Net,  $5.00 

ADDYMAN,  F,  T.  Practical  X-Ray  Work.  Illustrated.  8 vo.,  cloth,  200 
pp Net,  $4.00 

ALEXANDER,  J.  H.  Elementary  Electrical  Engineering  in  Theory  and 
Practice.  A  class-book  for  junior  and  senior  students  and  working 
electricians.  Illustrated.  12mo.,  cloth,  208  pp $2.00 

ANDERSON,  GEO.  L.  Handbook  for  the  Use  of  Electricians  in  the 
operation  and  care  of  Electrical  Machinery  and  Apparatus  of  the 
United  States  Seacoast  Defenses.  Prepared  under  the  direction  of 
Lieut.-General  Commanding  the  Army.  Illustrated  8vo.,  cloth,  161 
pp $3.00 

ARNOLD,  E.  Armature  Windings  of  Direct-Current  Dynamos.  Exten- 
sion and  Application  of  a  general  Winding  Rule.  Translated  from 
the  original  German  by  Francis  B.  DeGress.  Illustrated.  8vo. 
cloth,  124  pp $2.00 


ASHE,  S.  W.  Electricity  Experimentally  and  Practically  Applied. 
422  illustrations.  12mo.,  cloth,  375  pp Net,  $2.00 

ASHE,  S.  W.,  and  KEILEY,  J.  D.  Electric  Railways  Theoretically  and 
Practically  Treated.  Illustrated.  12mo.,  cloth. 

Vol.  I.     Rolling  Stock.     Second  Edition.     285  pp Net,  $2 . 50 

Vol.  II.     Substations  and  Distributing  Systems.     296  pp Net,  $2 . 50 

ATKINSON,  A.  A.  Electrical  and  Magnetic  Calculations.  For  the  use 
of  Electrical  Engineers  and  others  interested  in  the  Theory  and 
Application  of  Electricity  and  Magnetism.  Third  Edition,  revised. 
Illustrated.  12mo.,  cloth,  310  pp Net,  $1 . 50 

ATKINSON,  PHILIP.  The  Elements  of  Dynamic  Electricity  and  Mag- 
netism. Fourth  Edition.  Illustrated.  12mo.,  cloth,  405  pp.  .$2.00 

Elements  of  Electric  Lighting,  including  Electric  Generation,  Measure- 
ment, Storage,  and  Distribution.  Tenth  Edition,  fully  revised  and  new 
matter  added.  Illustrated.  12mo.,  cloth,  280  pp $1 .50 

Power  Transmitted  by  Electricity  and  Applied  by  the  Electric  Motor, 
including  Electric  Railway  Construction.  Illustrated.  Fourth  Edition, 
fully  revised  and  new  matter  added.  12mo.,  cloth,  241  pp $2.00 

AYRTON,  HERTHA.  The  Electric  Arc.  Illustrated.  8vo.,  cloth,  479 
pp Net,  $5.00 

AYRTON,  W.  E.  Practical  Electricity.  A  Laboratory  and  Lecture 
Course.  Illustrated.  12mo.,  cloth,  643  pp $2 .00 

BAKER,  J.  T.  The  Telegraphic  Transmission  of  Photographs.  63 
illustrations.  12mo.,  cloth,  155  pp Net,  $1.25 

BEDELL,  FREDERICK.  Direct  and  Alternating  Current  Testing. 
Assisted  by  C.  A.  Pierce.  Illustrated.  8vo.,  cloth.  250  pp.,  Net,  $2.00 

BEDELL,  F.  &  CREHORE,  ALBERT  C.  Alternating  Currents.  An 
analytical  and  graphical  treatment  for  students  and  engineers. 
Fifth  Edition.  112  illustrations.  8vo.,  cloth,  325  pp. .  .Net,  $2.50 

BIGGS,  C.  H.  W.  First  Principles  of  Electricity  and  Magnetism.  Illus- 
trated. 12mo.,  cloth,  495  pp .$2 .00 

BONNEY,  G.  E.  The  Electro-Plater's  Hand  Book.  A  Manual  for  Ama- 
teurs and  Young  Students  of  Electro-Metallurgy.  Fourth  Edition, 
enlarged.  61  Illustrations.  12mo.,  cloth,  208  pp $1 .20 


BOTTONE,  S.  R.  Magnetos  For  Automobilists;  How  Made  and  How  Used. 
A  handbook  of  practical  instruction  on  the  manufacture  and  adapta- 
tion of  the  magneto  to  the  needs  of  the  motorist.  Second  Edition, 

enlarged.     52  illustrations.     12mo.,  cloth,  118  pp Net,  $1 .00 

Electric  Motors,  How  Made  and  How  Used.  Illustrated.  12mo.,  cloth, 
168  pp 75  cents 

BOWKER,  WM.  R.  Dynamo,  Motor,  and  Switchboard  Circuits  for  Elec- 
trical Engineers:  a  practical  book  dealing  with  the  subject  of  Direct, 
Alternating,  and  Polyphase  Currents.  Second  Edition,  greatly 
enlarged,  130  illustrations.  8vo.,  cloth,  180  pp Net,  $2.50 

CARTER,  E.  T.  Motive  Power  and  Gearing  for  Electrical  Machinery;  a 
treatise  on  the  theory  and  practice  of  the  mechanical  equipment  of 
power  stations  for  electric  supply  and  for  electric  traction.  Second 
Edition,  revised.  Illustrated.  Svo.,  cloth,  700  pp .  .  ^Net,  $5.00 

CHILD,  CHAS.  T.  The  How  and  Why  of  Electricity :  a  book  of  informa- 
tion for  non-technical  readers,  treating  of  the  properties  of  Elec 
tricity,  and  how  it  is  generated,  handled,  controlled,  measured,  and 
set  to  work.  Also  explaining  the  operation  of  Electrical  Apparatus 
Illustrated.  8vo.,  cloth,  140  pp $1 .00 

CLARK,  D.  K.  Tramways,  Their  Construction  and  Working.  Second 
Edition.  Illustrated.  8vo.,  cloth,  758  pp $9 .00 

COOPER,  W.  R.     Primary  Batteries:    their  Theory,  Construction,  and  Use 

131  Illustrations.     8vo.,  cloth,  324  pp Net,  $4 .00 

The  Electrician  Primers.  Being  a  series  of  helpful  primers  on  electrical 
subjects,  for  use  of  students,  artisans,  and  general  readers.  Second 
Edition.  Illustrated.  Three  volumes  in  one.  8vo.,  cloth .  .  Net,  $5 . 00 

Vol.  I.— Theory Net,  $2.00 

Vol.  II. — Electric  Traction,  Lighting  and  Power Net,  $3 .00 

Vol.  IH.— Telegraphy,  Telephony,  etc Net,  $2  00 

CROCKER,  F.  B.  Electric  Lighting.  A  Practical  Exposition  of  the  Art 
for  the  use  of  Electricians,  Students,  and  others  interested  in  the 
Installation  or  Operation  of  Electric-Lighting  Plants. 

Vol.  I. — The  Generating  Plant.     Seventh  Edition,  entirely  revised.     Ulus 
trated.     8vo.,  cloth,  482  pp $3 .00 

Vol.  II. — Distributing  System  and  Lamps.     Sixth  Edition.     Illustrated 
8vo.,  cloth,  505  pp $3 .00 

CROCKER,  F.  B.,  and  ARENDT,  M.  Electric  Motors:  Their  Action, 
Control,  and  Application.  160  illustrations.  8vo.,  cloth,  296  pp.. 

Net,  $2.50 


CROCKER,  F.  B.,  and  WHEELER,  S.  S.  The  Management  of  Electrical 
Machinery.  Being  a  thoroughly  revised  and  rewritten  edition  of  the 
authors'  "Practical  Management  of  Dynamos  and  Motors." 
Seventh  Edition.  Illustrated.  16mo.,  cloth,  232  pp Net,  $1 .00 

CUSHING,  H.  C.,  Jr.  Standard  Wiring  for  Electric  Light  and  Power. 
Illustrated.  16mo.,  leather,  156  pp $1 .00 

DAMES,  F.  H.  Electric  Power  and  Traction.  Illustrated.  8vo.,  cloth, 
293  pp.  (Van  Nostrand's  Westminster  Series.) Net,  $2 .00 

DAWSON,  PHILIP.  Electric  Traction  on  Railways.  610  Illustrations. 
8vo.,  half  leather,  891  pp Net,  $9.00 

DEL  MAR,  W.  A.  Electric  Power  Conductors.  69  illustrations.  8vo., 
cloth,  330  pp Net,  $2 .00 

DIBDIN,  W.  J.  Public  Lighting  by  Gas  and  Electricity.  With  many  Tables, 
Figures,  and  Diagrams.  Illustrated.  8vo.,  cloth,  537  pp. Net,  $8.00 

DINGER,  Lieut.  H.  C.  Handbook  for  the  Care  and  Operation  of  Naval 
Machinery.  Second  Edition.  124  Illustrations.  16mo.,  cloth, 
302  pp Net,  $2.00 

DYNAMIC  ELECTRICITY:  Its  Modern  Use  and  Measurement,  chiefly 
in  its  application  to  Electric  Lighting  and  Telegraphy,  including: 
1 .  Some  Points  in  Electric  Lighting,  by  Dr.  John  Hopkinson.  2.  On 
the  Treatment  of  Electricity  for  Commercial  Purposes,  by  J.  N.  Shool- 
bred.  3.  Electric-Light  Arithmetic,  by  R.  E.  Day,  M.E.  Fourth 
Edition.  Illustrated.  16mo.,  boards,  166  pp.  (No.  71  Van  Nos- 
trand's Science  Series.) 50  cents 

EDGCUMBE,  K.  Industrial  Electrical  Measuring  Instruments.  Illus- 
trated. 8vo.,  cloth,  227  pp Net,  $2 .50 

ERSKINE-MURRAY,  J.  A  Handbook  of  Wireless  Telegraphy :  Its  Theory 
and  Practice.  For  the  use  of  electrical  engineers,  students,  and 
operators.  Second  Edition,  revised  and  enlarged.  180  Illustrations. 
8vo.,  cloth,  388  pp Net,  $3.50 

Wireless  Telephones   and  How   they  Work.     Illustrated.     16mo., 

cloth,  75  pp $1.00 

EWTNG,  J.  A.  Magnetic  Induction  in  Iron  and  other  Metals.  Third 
Edition,  revised.  Illustrated.  8vo.,  cloth,  393  pp Net,  $4.00 

FISHER,  H.  K.  C.,  and  DARBY,  W.  C.  Students'  Guide  to  Submarine  Cable 
Testing.  Third  Edition,  new,  enlarged.  Illustrated.  8vo.,  cloth, 
326 pp Net,  $3.50 


FLEMING,  J.  A.,  Prof.  The  Alternate-Current  Transformer  in  Theory 
and  Practice. 

Vol.I.:  The  Induction  of  Electric  Currents.  Fifth  Issue.  Illustrated. 
8vo.,  cloth,  641  pp Net,  $5 .00 

Vol.  II. :  The  Utilization  of  Induced  Currents.  Third  Issue.  Illus- 
trated. 8vo.,  cloth,  587  pp Net,  $5.00 

Handbook  for  the  Electrical  Laboratory  and  Testing  Room.  Two  Vol- 
umes. Illustrated.  8vo..  cloth,  1160  pp.  Each  vol Net,  $5.00 

FOSTER,  H.  A.  With  the  Collaboration  of  Eminent  Specialists.  Electri- 
cal Engineers'  Pocket  Book.  A  handbook  of  useful  data  for  Elec- 
tricians and  Electrical  Engineers.  With  innumerable  Tables,  Dia- 
grams, and  Figures.  The  most  complete  book  of  its  kind  ever  pub- 
lished, treating  of  the  latest  and  best  Practice  in  Electrical  Engineer- 
ing, sixth  Edition,  completely  revised  and  enlarged.  Fully  Illustrated. 
Pocket  Size.  Leather.  Thumb  Indexed.  1636  pp $5.00 

FOWLE,  F.  F.  The  Protection  of  Railroads  from  Overhead  Trans- 
mission Line  Crossings.  35  illustrations.  12mov  cloth,  76  pp. 

Net,  $1.50. 

GANT,  L.  W.  Elements  of  Electric  Traction  for  Motormen  and  Others. 
Illustrated  with  Diagrams.  8vo.,  cloth,  217  pp Net,  $2 .50 

GERHARDI,  C.  H.  W.  Electricity  Meters;  their  Construction  and  Man- 
agement. A  practical  manual  for  engineers  and  students.  Illus- 
trated. 8vo.,  cloth,  337  pp Net,  $4 .00 

GORE,  GEORGE.  The  Art  of  Electrolytic  Separation  of  Metals  (Theoret- 
ical and  Practical).  Illustrated.  8vo.,  cloth,  295  pp Net,  $3 . 50 

GRAY,  J.  Electrical  Influence  Machines :  Their  Historical  Development 
and  Modern  Forms.  With  Instructions  for  making  them.  Second 
Edition,  revised  and  enlarged.  With  105  Figures  and  Diagrams. 
12mo.,  cloth,  296  pp $2.00 

GROTH,  L.  A.  Welding  and  Cutting  Metals  by  Aid  of  Gases  or 
Electricity.  124  illustrations.  8vo.,  cloth,  280  pp Net,  $3.00 

HALLER,  G.  F.  and  CUNNINGHAM,  E.  T.  The  Tesla  High  Frequency 
Coil;  its  construction  and  uses.  12mo.,  cloth,  56  illustrations,  130 
pp In  Press 

HAMMER,  W.  J.  Radium,  and  Other  Radio  Active  Substances;  Polo- 
nium, Actinium,  and  Thorium.  With  a  consideration  of  Phospho- 
rescent and  Fluorescent  Substances,  the  properties  and  applications 
of  Selenium,  and  the  treatment  of  disease  by  the  Ultra- Violet  Light. 
With  Engravings  and  Plates.  Svo.,  cloth,  72  pp $1 .00 


HARRISON,  N.  Electric  Wiring  Diagrams  and  Switchboards.  Illus- 
trated. 12mo.,  cloth,  272  pp $1 .50 

HASKINS,  C.  H.  The  Galvanometer  and  its  Uses.  A  Manual  for  Elec- 
tricians and  Students.  Fifth  Edition,  revised.  Illustrated.  16mo., 
morocco,  75  pp $1 . 50 

HAWKINS,  C.  C.,  and  WALLIS,  F.  The  Dynamo :  Its  Theory,  Design, 
and  Manufacture.  Fourth  Edition,  revised  and  enlarged.  190  Illustra- 
tions. 8vo.,  cloth,  925  pp $3 .00 

HAY,  ALFRED.  Principles  of  Alternate-Current  Working.  Second  Edition. 

Illustrated.  12mo.,  cloth,  390  pp $2 .00 

Alternating  Currents;  their  theory,  generation,  and  transformation. 
Second  Edition.  191  Illustrations.  8vo.,  cloth,  319  pp .  .  .  Net,  $2 . 50 
An  Introductory  Course  of  Continuous-Current  Engineering.  Illus- 
trated. 8vo.,  cloth,  327  pp Net,  $2.50 

HEAVISIDE,  O.  Electromagnetic  Theory.  Two  Volumes  with  Many 
Diagrams.  8vo.,  cloth,  1006  pp.  Each  Vol Net,  $5 .00 

HEDGES,  K.  Modern  Lightning  Conductors.  An  illustrated  Supple- 
ment to  the  Report  of  the  Research  Committee  of  1905,  with  notes 
as  to  methods  of  protection  and  specifications.  Illustrated.  8vo., 
cloth,  119  pp Net,  $3.00 

HOBART,  H.  M.  Heavy  Electrical  Engineering.  Illustrated.  8vo., 
cloth,  338  pp Net,  $4.50 

Electricity.  A  text-book  designed  in  particular  for  engineering 

students.  115  illustrations.  43  tables.  8vo.,  cloth,  266  pp.,  Net,  $2 .00 

HOBBS,  W.  R.  P.  The  Arithmetic  of  Electrical  Measurements.  With 
numerous  examples,  fully  worked.  Twelfth  Edition.  12mo.,  cloth, 
126  pp 50  cents 

HOMANS,  J.  E.  A  B  C  of  the  Telephone.  With  269  Illustrations.  12mo., 
cloth,  352  pp SI  .00 

HOPKINS,  N.  M.  Experimental  Electrochemistry,  Theoretically  and  Prac- 
tically Treated.  Profusely  illustrated  with  130  new  d  rawings,  diagrams, 
and  photographs,  accompanied  by  a  Bibliography.  Illustrated. 
8vo.,  cloth,  298  pp Net,  $3 .00 

HOUSTON,  EDWIN  J.  A  Dictionary  of  Electrical  Words,  Terms,  and 
Phrases.  Fourth  Edition,  rewritten  and  greatly  enlarged.  582  Illus- 
trations. 4to.,  cloth Net,  $7 . 00 

A  Pocket  Dictionary  of  Electrical  Words,  Terms,  and  Phrases.     12mo., 
cloth,  950  pp Net,  $2.50 


HUTCHINSCN,  R.  W.,  Jr.  Long-Distance  Electric  Power  Transmission : 
Being  a  Treatise  on  the  Hydro-Electric  Generation  of  Energy;  Its 
Transformation,  Transmission,  and  Distribution.  Second  Edition. 
Illustrated.  12mo.,  cloth,  350  pp Net,  $3 .00 

HUTCHINSON,  R.  W.,  Jr.  and  IHLSENG,  M.  C.  Electricity  in  Mining. 
Being  a  theoretical  and  practical  treatise  on  the  construction, 
operation,  and  maintenance  of  electrical  mining  machinery.  12mo., 
cloth In  Press 

INCANDESCENT  ELECTRIC  LIGHTING.  A  Practical  Description  of 
the  Edison  System,  by  H.  Latimer.  To  which  is  added:  The  Design 
and  Operation  of  Incandescent  Stations,  by  C.  J.  Field;  A  Descrip- 
tion of  the  Edison  Electrolyte  Meter,  by  A.  E.  Kennelly;  and  a 
Paper  on  the  Maximum  Efficiency  of  Incandescent  Lamps,  by  T.  W. 
Howell.  Fifth  Edition.  Illustrated.  16mo.,  cloth,  140  pp.  (No. 
57  Van  Nostrand's  Science  Series.) 50  cents 

INDUCTION  COILS:  How  Made  and  How  Used.  Eleventh  Edition. 
Illustrated.  16mo.,  cloth,  123  pp.  (No.  53  Van  Nostrand's  Science 
Series.) 50  cents 

JEHL,  FRANCIS.  The  Manufacture  of  Carbons  for  Electric  Lighting 
and  other  purposes.  Illustrated  with  numerous  Diagrams,  Tables, 
and  Folding  Plates.  8vo.,  cloth,  232  pp Net,  $4 . 00 

JONES,  HARRY  C.     The  Electrical  Nature  of  Matter  and  Radioactivity. 

Second  Edition,  revised  and  enlarged.     12mo.,  cloth,  218  pp.  .$2.00 

KAPP,  GISBERT.  Electrical  Transmission  of  Energy  and  its  Transforma- 
tion, Subdivision,  and  Distribution.  A  Practical  Handbook.  Fourth 
Edition,  thoroughly  revised.  Illustrated.  12mo.,  cloth,  445  pp .  .  $3 . 50 

Alternate-Current  Machinery.  Illustrated.  16mo.,  cloth,  190  pp.  (No. 
96  Van  Nostrand's  Science  Series.) 50  cents 

Dynamos,  Alternators  and  Transformers.  Illustrated.  8vo.,  cloth,  507 
pp v $4.00 

KELSEY,  W.  R.  Continuous-Current  Dynamos  and  Motors,  and  their 
Control;  being  a  series  of  articles  reprinted  from  the  "Practical 
Engineer,"  and  completed  by  W.  R.  Kelsey,  B.Sc.  With  Tables, 
Figures,  and  Diagrams.  8vo.,  cioth,  439  pp $2 . 50 

KEMPE,  H.  R.  A  Handbook  of  Electrical  Testing.  Seventh  Edition, 
revised  and  enlarged.  Illustrated.  8vo.,  cloth,  706  pp. .  .Net,  $6.00 


KENNEDY,   R.     Modern    Engines   and   Power   Generator.     Illustrated. 

8vo.,  cloth,  5  vols.     Each.     The  set,  $15.00 $3 . 50 

Electrical  Installations  of  Electric  Light,  Power,  and  Traction  Machinery. 
Illustrated.     8vo.,  cloth,  5  vols.     Each $3 . 50 

KENNELLY,  A.  E.  Theoretical  Elements  of  Electro-Dynamic  Machinery. 
Vol.  I.  Illustrated.  8vo.,  cloth,  90  pp $1 . 50 

KERSHAW,  J.  B.  C.     The  Electric  Furnace  in  Iron  and  Steel  Production. 

Illustrated.     8vo.,  cloth,  74  pp Net,  $1 . 50 

Electrometallurgy.     Illustrated.     8vo.,     cloth,     303    pp.     (Van     Nos- 
trand's  Westminster  Series.) Net,  $2 .00 

KINZBRUNNER,  C.     Continuous-Current  Armatures;    their  Winding  and 

Construction.     79  Illustrations.     8vo.,  cloth,  80  pp Net,  $1 .50 

Alternate-Current  Windings;    their  Theory  and  Construction.     89  Illus- 
trations.    8vo.,  cloth,  80  pp Net,  SI  •  50 

KOESTER,  F.  Hydroelectric  Developments  and  Engineering.  A  practi- 
cal and  theoretical  treatise  on  the  development,  design,  construction, 
equipment  and  operation  of  hydroelectric  transmission  plants.  500 
illustrations.  4to.,  cloth,  475  pp Net,  $5.00 

Steam-Electric  Power  Plants.     A  practical  treatise  on  the  design  of 

central   light  and  power  stations  and  their  economical   construction 
and  operation.      Fully  Illustrated.     4to.,   cloth,  455  pp . . Net,  $5 . 00 

LARNER,  E.  T.  The  Principles  of  Alternating  Currents  for  Students  of 
Electrical  Engineering.  Illustrated  with  Diagrams.  12mo.,  cloth, 
144  pp Net,  $1 .50 

LEMSTROM,  S.  Electricity  in  Agriculture  and  Horticulture.  Illustrated. 
8vo.,  cloth Net,  $1 .50 

LIVERMORE,  V.  P.,  and  WILLIAMS,  J,  How  to  Become  a  Competent 
Motorman :  Being  a  practical  treatise  on  the  proper  method  of  oper- 
ating a  street-railway  motor-car;  also  giving  details  how  to  over- 
come certain  defects.  Second  Edition.  Illustrated.  16mo.,  cloth, 
247  pp Net,  $1 .00 

LOCKWOOD,  T.  D.  Electricity,  Magnetism,  and  Electro-Telegraphy.  A 
Practical  Guide  and  Handbook  of  General  Information  for  Electri- 
cal Students,  Operators,  and  Inspectors.  Fourth  Edition.  Illus- 
trated. 8vo.,  cloth,  374  pp $2.50 


LODGE,  OLIVER  J.  Signalling  Across  Space  Without  Wires:  Being  a 
description  of  the  work  of  Hertz  and  his  successors.  Third  Edition. 
Illustrated.  8vo.,  cloth Net,  $2 .00 

LORING,  A.  E.  A  Handbook  of  the  Electro-Magnetic  Telegraph. 
Fourth  Edition,  revised.  Illustrated.  16mo.,  cloth,  116  pp.  (No. 
39  Van  Nostrand's  Science  Series.) 50  cents 

LUPTON,  A.,  PARR,  G.  D.  A.,  and  PERKIN,  H.  Electricity  Applied  to 
Mining.  Second  Edition.  With  Tables,  Diagrams,  and  Folding 
Plates.  8vo.,  cloth,  320  pp Net,  $4.50 

MAILLOUX,  C.  0.  Electric  Traction  Machinery.  Illustrated.  8vo., 
cloth In  Press 

MANSFIELD,  A.  N.  Electromagnets:  Their  Design  and  Construction. 
Second  Edition.  Illustrated.  16mo.,  cloth,  155  pp.  (No.  64  Van 
Nostrand's  Science  Series.) 50  cents 

MASSIE,  W.  W.,  and  UNDERBILL,  C.  R.  Wireless  Telegraphy  and 
Telephony  Popularly  Explained.  With  a  chapter  by  Nikola  Tesla. 
Illustrated.  12mo.,  cloth,  82  pp Net,  $1 .00 

MAURICE,  W.  Electrical  Blasting  Apparatus  and  Explosives,  with 
special  reference  to  colliery  practice.  Illustrated.  8vo.,  cloth, 
167  pp Net,  $3.50 

The  Shot  Filer's  Guide.     A  practical  manual  on  blasting  and  the 

prevention  of v  blasting  accidents.       78  illustrations.     8vo.,    cloth, 
212  pp Net,  $1.50 

MAVER,  WM.,  Jr.  American  Telegraphy  and  Encyclopedia  of  the  Tele- 
graph Systems,  Apparatus,  Operations.  Fifth  Edition,  revised.  450 
Illustrations.  8vo.,  cloth,  656  pp Net,  $5.00 

MONCKTON,  C.  C.  F.  Radio  Telegraphy.  173  Illustrations.  8vo., 
cloth,  272  pp.  (Van  Nostrand's  Westminster  Series.) Net,  $2.00 

MORGAN,  ALFRED  P.  Wireless  Telegraph  Construction  for  Amateurs. 
153  illustrations.  12mo.,  cloth,  220  pp Net,  $1.50 

MUNRO,  J.,  and  JAMIESON,  A.  A  Pocket-Book  of  Electrical  Rules  and 
Tables  for  the  Use  of  Electricians,  Engineers,  and  Electrometallurgists. 
Eighteenth  Revised  Edition.  32mo.,  leather,  735  pp $2.50 


NIPHER,  FRANCIS  E.  Theory  of  Magnetic  Measurements.  With  an 
Appendix  on  the  Method  of  Least  Squares.  Illustrated.  12mo., 
cloth,  94  pp $1 .00 

NOLL,  AUGUSTUS.  How  to  Wire  Buildings.  A  Manual  of  the  Art  of 
Interior  Wiring.  Fourth  Edition.  Illustrated.  12mo.,  cloth, 
165  pp $1 .50 

OHM,  G.  S.  The  Galvanic  Circuit  Investigated  Mathematically.  Berlin, 
1827.  Translated  by  William  Francis.  With  Preface  and  Notes 
by  Thos.  D.  Lockwood.  Second  Edition.  Illustrated.  16mo.,  cloth, 
269  pp.  (No.  102  Van  Nostrand's  Science  Series.) 50  cents 

OLSSON,  ANDREW.  Motor  Control  as  used  in  Connection  with  Turret 
Turning  and  Gun  Elevating.  (The  Ward  Leonard  System.)  13 
illustrations.  12mo.,  paper,  27  pp.  (U.  S.  Navy  Electrical  Series 
No.  1.) Net,  .50 

OUDIN,  MAURICE  A.  Standard  Polyphase  Apparatus  and  Systems. 
Fifth  Edition,  revised.  Illustrated  with  many  Photo-reproductions, 
Diagrams,  and  Tables.  8vo.,  cloth,  369  pp Net,  $3.00 

PALAZ,  A.  Treatise  on  Industrial  Photometry.  Specially  applied  to 
Electric  Lighting.  Translated  from  the  French  by  G.  W.  Patterson, 
Jr.,  and  M.  R.  Patterson.  Second  Edition.  Fully  Illustrated. 
8vo.,  cloth,  324  pp $4 .00 

PARR,  G.  D.  A.  Electrical  Engineering  Measuring  Instruments  for  Com- 
mercial and  Laboratory  Purposes.  With  370  Diagrams  and  Engrav- 
ings. 8vo.,  cloth,  328  pp Net,  $3.50 

PARSHALL,  H.  F.?  and  HOBART,  H.  M.  Armature  Windings  of  Electric 
Machines.  Third  Edition.  With  140  full-page  Plates,  65  Tables, 
and  165  pages  of  descriptive  letter-press.  4to.,  cloth,  300  pp.  .$7 .50 

Electric  Railway  Engineering.  With  437  Figures  and  Diagrams 
and  many  Tables.  4to.,  cloth,  475  pp Net,  $10.00 

Electric  Machine  Design.  Being  a  revised  and  enlarged  edition  of 
"Electric  Generators."  648  Illustrations.  4to.,  half  morocco,  601 
pp Net,  $12.50 

PERRINE,  F.  A.  C.  Conductors  for  Electrical  Distribution :  Their  Manu- 
facture and  Materials,  the  Calculation  of  Circuits,  Pole-Line  Construc- 
tion, Underground  Working,  and  other  Uses.  Second  Edition.  Illus- 
trated. 8vo.,  cloth,  287  pp Net,  $3.50 


POOLE,  C.  P.  The  Wiring  Handbook  with  Complete  Labor-saving  Tables 
and  Digest  of  Underwriters'  Rules.  Illustrated.  12mo.,  leather, 
85  pp Net,  $1 .00 

POPE,  F.  L.  Modern  Practice  of  the  Electric  Telegraph.  A  Handbook 
for  Electricians  and  Operators.  Seventeenth  Edition.  Illustrated. 
8vo.,  cloth,  234  pp :  . ; .  .-.".^ ';. •.' $1 .50 

RAPHAEL,  F.  C.  Localization  of  Faults  in  Electric  Light  Mains.  Second 
Edition,  revised.  Illustrated.  Svo.,  cloth,  205  pp Net,  $3.00 

RAYMOND,  E.  B.  Alternating-Current  Engineering,  Practically  Treated. 
Third  Edition,  revised.  With  many  Figures  and  Diagrams.  8vo., 
cloth,  244  pp : Net,  $2 .50 

RICHARDSON,  S.  S.  Magnetism  and  Electricity  and  the  Principles  of  Elec- 
trical Measurement.  Illustrated.  12mo.,  cloth,  596  pp.  .Net,  $2.00 

ROBERTS,  J.  Laboratory  Work  in  Electrical  Engineering — Preliminary 
Grade.  A  series  of  laboratory  experiments  for  first-  and  second-year 
students  in  electrical  engineering.  Illustrated  with  many  Diagrams. 
8vo.,  cloth,  218  pp ......  r... Net,  $2 . 00 

ROLLINS,  W.  Notes  on  X-Light.  Printed  on  deckle  edge  Japan  paper. 
400  pp.  of  text,  152  full-page  plates.  8vo.,  cloth Net,  $7 . 50 

RUHMER,  ERNST.  Wireless  Telephony  in  Theory  and  Practice.  Trans- 
lated from  the  German  by  James  Erskine-Murray.  Illustrated. 
8vo.,  cloth,  224  pp Net,  $3.50 

RUSSELL,  A.  The  Theory  of  Electric  Cables  and  Networks.  71  Illus- 
trations. 8vo.,  cloth,  275  pp Net,  $3.00 

SALOMONS,  DAVID.     Electric-Light  Installations.     A   Practical   Hand- 
book.    Illustrated.     12mo.,  cloth. 
Vol.1.:    Management  of  Accumulators.     Ninth  Edition.     178  pp.  $2. 50 

Vol.  II.:    Apparatus.     Seventh  Edition.     318  pp $2 .25 

Vol.  III.:    Application.     Seventh  Edition.     234  pp . . .$1 .50 

SCHELLEN,  H.  Magneto-Electric  and  Dynamo-Electric  Machines.  Their 
Construction  and  Practical  Application  to  Electric  Lighting  and  the 
Transmission  of  Power.  Translated  from  the  Third  German  Edition 
by  N.  S.  Keith  and  Percy  Neymann.  With  Additions  and  Notes 
relating  to  American  Machines,  by  N.  S.  Keith.  Vol.  I.  With 
353  Illustrations.  Third  Edition.  8vo.,  cloth,  518  pp $5 .00 


SEVER,  G.  F.  Electrical  Engineering  Experiments  and  Tests  on  Direct- 
Current  Machinery.  Second  Edition,  enlarged.  With  Diagrams  and 
Figures.  8vo.,  pamphlet,  75  pp Net,  $1 .00 

SEVER,  G.  F.,  and  TOWNSEND,  F.  Laboratory  and  Factory  Tests  in 
Electrical  Engineering.  Second  Edition,  revised  and  enlarged.  Illus- 
trated. 8vo.,  cloth,  269  pp Net,  $2 . 50 

SEWALL,  C.  H.  Wireless  Telegraphy.  With  Diagrams  and  Figures. 
Second  Edition,  corrected.  Illustrated .  8vo.,  cloth,  229  pp .  .  Net,  $2 . 00 

Lessons  in  Telegraphy.     Illustrated.     12mo.,  cloth,  104  pp.  .Net,  $1 .00 

SEWELL,  T.  Elements  of  Electrical  Engineering.  Third  Edition, 
revised.  Illustrated.  8vo.,  cloth,  444  pp $3.00 

The  Construction  of  Dynamos  (Alternating  and  Direct  Current).  A 
Text-book  for  students,  engineering  contractors,  and  electricians-in- 
charge.  Illustrated.  8vo.,  cloth,  316  pp S3 .00 

SHAW,  P.  E.  A  First-Year  Course  of  Practical  Magnetism  and  Electricity. 
Specially  adapted  to  the  wants  of  technical  students.  Illustrated. 
8vo.,  cloth,  6G  pp.  interleaved  for  note  taking Net,  $1 .00 

SHELDON,  S.,  and  HAUSMANN,    E.      Dynamo-Electric  Machinery:    Its 

Construction,  Design,  and  Operation. 

Vol.  I.:  Direct-Current  Machines.  Eighth  Edition,  completely  re-written. 
Illustrated.  8vo.,  cloth,  310  pp Net,  $2.50 

SHELDON,  S.,  MASON,  H.,  and  HAUSMANN,  E.  Alternating-Current 
Machines:  Being  the  second  volume  of  "Dynamo-Electric 
Machinery;  its  Construction,  Design,  and  Operation."  With  many 
Diagrams  and  Figures.  (Binding  uniform  with  Volume  I.) 
Seventh  Edition,  rewritten.  8vo.,  cloth,  353  pp Net,  $2.50 

SLOANE,  T.  O'CONOR.  Standard  Electrical  Dictionary.  300  Illustra- 
tions. 12mo.,  cloth,  682  pp $3.00 

Elementary  Electrical  Calculations.  A  Manual  of  Simple  Engineer- 
ing Mathematics,  covering  the  whole  field  of  Direct  Current 
Calculations,  the  basis  of  Alternating  Current  Mathematics,  Net- 
works, and  typical  cases  of  Circuits,  with  Appendices  on  special 
subjects.  8vo.,  cloth.  Illustrated.  304  pp Net,  $2.00 

SNELL,  ALBION  T.  Electric  Motive  Power.  The  Transmission  and  Dis- 
tribution of  Electric  Power  by  Continuous  and  Alternating  Currents. 
With  a  Section  on  the  Applications  of  Electricity  to  Mining  Work. 
Second  Edition.  Illustrated.  8vo.,  cloth,  411  pp Net,  $4.00 


SODDY,  F.  Radio-Activity ;  an  Elementary  Treatise  from  the  Stand- 
point of  the  Disintegration  Theory.  Fully  Illustrated.  8vo.,  cloth, 
214  pp Net,  $3.00 

SOLOMON,  MAURICE.  Electric  Lamps.  Illustrated.  8vo.,  cloth.  (Van 
Nostrand's  Westminster  Series.) Net,  $2 .00 

STEWART,  A.  Modern  Polyphase  Machinery.  Illustrated.  12mo., 
cloth,  296  pp Net,  $2 .00 

SWINBURNE,  JAS.,  and  WORDINGHAM,  C.  H.  The  Measurement  of 
Electric  Currents.  Electrical  Measuring  Instruments.  Meters  for 
Electrical  Energy.  Edited,  with  Preface,  by  T.  Commerford  Martin. 
Folding  Plate  and  Numerous  Illustrations.  16mo,,  cloth,  241  pp. 
(No.  109  Van  Nostrand's  Science  Series.) 50  cents 

SWOOPE,  C.  WALTON.  Lessons  in  Practical  Electricity:  Principles, 
Experiments,  and  Arithmetical  Problems.  An  Elementary  Text- 
book. With  numerous  Tables,  Formula,  and  two  large  Instruction 
Plates.  Eleventh  Edition,  revised.  Illustrated.  8vo.,  cloth,  462  pp. 

Net,  $2.00 

THOM,  C.,  and  JONES,  W.  H.  Telegraphic  Connections,  embracing  recent 
methods  in  Quadruplex  Telegraphy.  20  Colored  Plates.  8vo., 
cloth,  59  pp $1 .50 

THOMPSON,  S.  P.  Dynamo-Electric  Machinery.  With  an  Introduction 
and  Notes  by  Frank  L.  Pope  and  H.  R.  Butler.  Fully  Illustrated. 
16mo.,  cloth,  214  pp.  (No.  66  Van  Nostrand's  Science  Series.) 

50  cents 

Recent  Progress  in  Dynamo-Electric  Machines.  Being  a  Supplement  to 
"Dynamo-Electric  Machinery."  Illustrated.  16mo.,  cloth,  113  pp. 
(No.  75  Van  Nostrand's  Science  Series.) 50  cents 

TOWNSEND,  FITZHUGH.  Alternating  Current  Engineering.  Illus- 
trated. 8vo.,  paper,  32  pp Net,  75  cents 

UNDERBILL,  C.  R.  Solenoids,  Electromagnets  and  Electromagnetic 
Windings.  218  Illustrations.  12mo.,  cloth,  345  pp Net,  $2.00 

URQUHART,  J.  W.  Dynamo  Construction.  A  Practical  Handbook  for 
the  use  of  Engineer  Constructors  and  Electricians  in  Charge.  Illus- 
trated. 12mo.,  cloth $3.00 


Electric  Ship-Lighting.  A  Handbook  on  the  Practical  Fitting  and  Run- 
ning of  Ship's  Electrical  Plant,  for  the  use  of  Ship  Owners  and  Build- 
ers, Marine  Electricians,  and  Sea-going  Engineers  in  Charge.  88 
Illustrations.  12mo.,  cloth,  308  pp $3 .00 

Electric-Light  Fitting.  A  Handbook  for  Working  Electrical  Engineers, 
embodying  Practical  Notes  on  Installation  Management.  Second 
Edition,  with  additional  chapters.  With  numerous  Illustrations. 
12mo.,  cloth $2 . 00 

Electroplating.  A  Practical  Handbook.  Fifth  Edition.  Illustrated. 
12mo.,  cloth,  230  pp $2 .00 

Electrotyping.     Illustrated.     12mo.,  cloth,  228  pp $2.00 

WADE,  E.  J.  Secondary  Batteries :  Their  Theory,  Construction,  and  Use. 
Second  Edition,  corrected.  265  Illustrations.  8vo.,  cloth,  501  pp. 

Net,  $4.00 

WADSWORTH,  C.  Electric  Battery  Ignition.  15  Illustrations.  16mo. 
paper In  Press 

WALKER,  FREDERICK.  Practical  Dynamo-Building  for  Amateurs. 
How  to  Wind  for  any  Output.  Third  Edition.  Illustrated.  16mo., 
cloth,  104  pp.  (No.  98  Van  Nostrand's  Science  Series.) 50  cents 

WALKER,  SYDNEY  F.  Electricity  in  Homes  and  Workshops.  A 
Practical  Treatise  on  Auxiliary  Electrical  Apparatus.  Fourth  Edition. 
Illustrated.  12mo.,  cloth,  358  pp $2 .00 

Electricity  in  Mining.     Illustrated,     8vo.,  cloth,  385  pp $3.50 

WALLING,  B.  T.,  and  MARTIN,  JULIUS.  Electrical  Installations  of  the 
United  States  Navy.  With  many  Diagrams  and  Engravings.  8vo., 
cloth,  648  pp $6.00 

WALMSLEY,  R.  M.  Electricity  in  the  Service  of  Man.  A  Popular  and 
Practical  Treatise  on  the  Application  of  Electricity  in  Modern  Life. 
Illustrated.  8vo.,  cloth,  1208  pp Net,  $4.50 

WATT,  ALEXANDER.  Electroplating  and  Refining  of  Metals.  New 
Edition,  rewritten  by  Arnold  Philip.  Illustrated.  8vo.,  cloth,  677 

PP Net,  $4.50 

Electrometallurgy.  Fifteenth  Edition.  Illustrated.  12mo.,  cloth,  225 
PP $1 .00 


WEBB,  H.  L.  A  Practical  Guide  to  the  Testing  of  Insulated  Wires  and 
Cables.  Fifth  Edition.  Illustrated.  12mo.,  cloth,  118  pp $1 .00 

WEEKS,  R.  W.      The  Design   of  Alternate-Current  Transformer. 

New  Edition  in  Press 

WEYMOUTH,  F.  MARTEN.  Drum  Armatures  and  Commutators. 
(Theory  and  Practice.)  A  complete  treatise  on  the  theory  and  con- 
struction of  drum-winding,  and  of  commutators  for  closed-coil  arma- 
tures, together  with  a  full  resum6  of  some  of  the  principal  points 
involved  in  their  design,  and  an  exposition  of  armature  reactions 
and  sparking.  Illustrated.  8vo.,  cloth,  295  pp Net,  $3 .00 

WILKINSON,  H.  D.  Submarine  Cable  Laying,  Repairing  and  Testing. 
Second  Edition,  completely  revised.  313  Illustrations.  8vo.,  cloth? 
580  pp Net,  $6.00 

YOUNG,  J.  ELTON.  Electrical  Testing  for  Telegraph  Engineers.  Illus- 
trated. 8vo.,  cloth,  264  pp Net,  $4.00 

ZEIDLER,  J.,  and  LUSTGARTEN,  J.  Electric  Arc  Lamps:  Their  Princi- 
ples, Construction  and  Working.  160  Illustrations.  8vo.,  cloth, 
188  pp Net,  $2.00 


A  96=page  Catalog  of  Books  on  Electricity,  classified  by 
subjects,  will  be  furnished  gratis,  postage  prepaid, 
on  application. 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 
This  book  is  DUE  on  the  last  date  stamped  below. 

Fine  schedule:  25  cents  on  first  day  overdue 

50  cents  on  fourth  day  overdue 
One  dollar  on  seventh  day  overdue. 


>.;-:.,  A  ••.•• 


APR  16 


LD  21-100m-12,'46(A2012sl6)4120 


